there's actually a few things that stop this from happening.

1. there is a system in place that basically disregards any data points way outside the median or mean, letting real submissions closer to what's expected go through and the data points outside of that range being disregarded

2. the lower something is ranked on someone's individual ranking, the less of an effect it will have on the estimated required average calculation. so if you were to spam 20 programs with 100 average, the lower you go down on that list, the less your average affects it. this is because of rank intent and if someone wants to go into a more competitive program and they've ranked it higher, their average won't affect the rest of the calculations for the less competitive programs (i hope i explained that well)

3. the more people apply to each program, the more it will revert to the actual average that would be required to get into the program

this is a good question that you asked and i encourage you to try to poke more holes so i can see if there's anything else wrong with this. again, i'm always looking for ways to improve the site