I think philomastix made some great points, so I don't have much to add to what he said, though I will bring up a few additional thoughts.
Now, I'll preface this right off of the bat with the fact that I've only done missions work in East Asia, and never in Africa, so I'm sure there are different perspectives between the two that I'm completely unaware of, but I do think there are a few core things that would remain the same wherever one goes.
They are there for sure, and they're a big need. However much of that falls at the responsibility of the church, that has (in majority) shrugged this responsibility off, and said they'd rather have the state taking care of widows and orphans.
I mean, let's be honest, when was the last time you saw a congregation as a whole actively taking in widows and orphans the way the book of Acts and the rest of the New Testament tells us to? I'm not saying it DOESN'T happen, but it doesn't happen with nearly the faithfulness and selflessness as it SHOULD be happening. And the worst part of it is that this should be the place where we absolutely KNOW this is the responsibility of the church. We've had that truth in America since the foundation of our country.
Now one could argue (and I'd have no problem with this reasoning) that knowing this, it should incline one even more to stay and to take hold of this, and work in their local mission field. However, the Lord doesn't always work with each person the same way. Those in America have an abundance of people who should RIGHTLY KNOW that they have a responsibility to the world for the sake of Christ that they're not fulfilling. But what about Africa (as a whole)? What about Turkey? What about China, Indonesia, Thailand, Japan, India, etc.? They don't have an abundance of people who know how the church should be acting and are just refusing to do it. They don't have anything. They most likely have never heard the name of Jesus, and even if they have, they odds are they've never seen Him living and directing the life of a believer. Just by sheer odds alone they have never had any encounter with the living Christ.
I mean, basic analogy, but it's more or less a Light in the darkness sort of thing. There's plenty of light in the States. But what about the lands where darkness abounds? America has had so much light, and so much opportunity to do right, I don't have any problem with someone doing as the apostles did and "shaking the dust off of their feet" with America. But that's not everyone's calling. Some are equally called to stay and proclaim truth. We're not to be the judges of that. God will lead and direct each person where He would have them (if they'll listen and obey).
Also, following that thought, consider this. And this may sound extremely simple and rudimentary, but this is something huge, so REALLY consider this. In America, by statistics alone, it would be quite a rare occasion to encounter anyone who has never been prayed for. Be it by their parents, children, relative, co-workers, friends, acquaintances, whoever. Odds are, just by the massive number of people who at least profess to be Christians in America, every person you meet will have more than likely, had someone, at sometime, praying for their salvation.
Now lets switch gears. I'll use my locale for a reference point. Japan. Roughly around 130,000,000 people. Of that, professing Christians alone (and this is a liberal estimation, as it only refers to people who probably don't feel completely uncomfortable being called a Christian,(and the Japanese include Catholics, Mormons, JWs in this count as well)) we're only looking at 1%.
Now, for actual believing Christians by the Biblical estimation, I think the estimates are (and this is a generous estimate indeed, though just from what I've seen here I would have to say it's considerably lower) around 0.5% or less. So in essence, realistically we're looking at less than 650,000 professing Christians in the whole country. And based just in what ive seen, I personally think the number is stageringly smaller than even that estimate. Based on that alone, there's the high possibility that everyday, whenever I'm out in a sea of people here, the chances are quite high, that not a single person around me has ever had their salvation personally interceded for before God. That's a big deal. A REALLY BIG deal. In the States people aren't looking at those type of odds. But that's the reality of missions work in foreign countries. And the contrast is sharp.
I mean, we many times see people we know, and people we love get saved around us in the States, but I would venture to say that all these people have had someone bringing them (faithfully or infrequently, either way) before God in prayer. Who prays for the specific people in the type of places where I'm at though? And places like Africa? Prayer is the one weapon that Satan and all of the world have no means whatsoever to combat. They can stop building programs, attack our bodies, our health, our finances, our families, about anything we can think of. They can not stop nor combat our prayers though.
This is one of the major differences between working in America where there is need, and working in the darkness of a mission field (where there is also need). Each is called to the place that God would have them.
As far as people's motives and intentions go, only God knows. Now, I know exactly what has been brought up here about people just going for trips and stamping God's name on top of it. I personally see that all the time here. Short term missions group that in actuality are just a church group on vacation together. I'm well aware of such things, and they're far too common. However, even places like where I'm at have churches filled with people who also know the truth, yet don't really feel like acting on it, thus making the burden even more overwhelming. In such places, we'll take as many as God would send that have a heart for outreach, even if we think they could have been used where there from, because honestly, we have the same need, but not nearly as many laborers.
And all in all, the ones who come to the field with selfish ambitions usually get rooted out within the first year or so anyway. God can sort out people's intentions.
Hopefully this answers a bit of your initial inquiry. I'm a bit pressed for time, and wasn't able to get into specifics, but hopefully this makes a tad bit of sense.
