I'm sure many of them are sincere in their beliefs, though many more are likely just doing it for the moneys, like Joel Osteen.
Televangelism is the natural consequence of theology that denies the importance of ritual, tradition and ceremony, as well as community. If only the message and knowledge of Jesus matter, and that's all you need, why not just broadcast it to as many people as possible via TV?
Comfort might be the key here. Its simply easier to watch these things from the safety of your own house than it is to go to church and interact with people, and have judgement and expectations placed upon you by others. And again, what's the use in that, since the message is everything?
I think Televangelists will exists wherever Protestantism exists. That of course means mostly America and Latin America, though I'm sure there's a few in Europe as well, though likely outnumbered by Muslims, Catholics and the predominant group: Atheists (regardless of their "cultural faith). They'll rise up in China soon as the government lets off, and they are already pretty potent in Korea (the lower one).