Because bigger field tournaments take longer to complete and come with higher variance.
poker sites are not donating you their money, so if they guaranty to pay out 2.000$ for a 5$ buyin, then you should expect at least 400 entrances to that tournament. Whereas if its only 500$, then probably only 100 people will play.
Sure its not as exciting to win 120$ for taking down the small tournament as to win 360$ for taking down the big tournament, but you are going to win the small one 4 times as often give or take. And if your goal is to gradually build a
bankroll, then frequent small wins are actually more desirable than rare big ones.
This is not to say, that I wont play a 400 player MTT, but I certainly dont prefer it over a 100 player MTT. Also some MTT are way larger than that especially on a site like
pokerstars. And if it gets to like 4.000 players, then its almost like, you are spending 30-50% of the buyin to buy a lottery ticket. So very large MTTs are best avoided, if you play for profit.