Why are FCS teams "worse" today than 10-15 years ago?

Transfer portal and bigger schools poaching the better players from the division to go play with the big boys. In addition to NIL monies making the move even more tempting for the schools who don't have millions of dollars.

Granted this is not the sole reason for lower ratings by Sagarin this year or in recent years, but I'm confident it has a decent part to play in results.