You can play a *lot* of single table S&G's so your actual EV will match your results in a shorter amount of time (measured in months or years).
Single table S&G's pay the top 1/3 of players:
1st = 50%
2nd = 30%
3rd = 20%
So it's pretty easy for a good player to cash on a consistent basis.
MTT's will pay the top 10%, so you cash a lot less. Also, the big money is in the top places, so you have to go farther to get there. How many 7,800 player Sunday Million games would a skilled player have to win to get 1st place?
Now, MTT's have a massive payout at the end. So after many many losses and small wins, that player will eventually have a big win which will make up for those unprofitable/barely-profitable games.
So yeah, *eventually* a skilled MTT player will have a larger ROI than an equal S&G player. However, that S&G player can play ten times as many games. So they get to cash in on their EV a lot more than MTT players do.