Many megachurches preach a prosperity gospel, the idea that God wants us to be rich, and if you aren't, it's through some fault of your own. By tithing to the church as much money as possible, you show your faith in the Lord and it — according to the church — will return to you, the parishioner, a hundredfold.
Of course, only the pastors seem to be getting rich; many of them claim millions of dollars a year in salary, as well as perks like private jets or houses the size of hotels. The prosperity gospel has been blamed for the housing crisis of 2008 and criticized for preying on the poor, encouraging them to give to the church before paying rent or bills, telling them that the $65 million jets they're helping the pastor to purchase are God's will. Hashtag blessed.
One of the best cons ever invented, a good grifter can make millions using it. That is all these pastors are is good grifters.