I'm not sure that this is entirely true: While I don't know what might have happened behind the scenes, and this likely doesn't happen for a great many games, I've seen a number of interesting indie titles turn up on the channels run by Jim Sterling, TotalBiscuit, some of the Yogscast (more seldom), and others besides, I believe.
Definitely.
The youtubers who have build a name around good game reviews also have a reputation to uphold.
If they started to review too many games for the cash, they will lose in credibility, and in the end lose the position they worked so hard to get to.
They are also often very outspoken against payed reviews, and if doing them, are open with it.
It's another matter with the "silly youtubers", building a name around making entertaining stuff.
Most of the games are likely games they just want to play, because they are "in the buzz" right now, and it is therefore likely to draw viewers, but I'm convinced there is more and more payed content.