tithing. BYU can't dictate how people want to spend their money.
If this is a BYU football focused endowment than it would make what... 4-5%? Even the reported $100 million is correct than we are talking $4-5 million. Not sure that is divided equally but if it were divided with over 100 football players we are looking at less than $50K a piece. Does that move the needle? Sure but we are not talking about life changing money. The endowment sounds ridiculous collectively but if you spend it than it is all gone so it is likely the type of things that fans will pay slowly into or drop one time lump sums into, knowing their money will make a difference over time.
Even with my small pockets I would feel better about giving a BYU NIL fund $1000, knowing it would make a small impact over time rather than get burned in one year. As an example, I also wouldn't want BYU to tell me it was for football and then bait and switch it to some kind of grant to help an a needy student.
To tell you the truth I have no idea how it works but I very much doubt that BYU fans have ponied up an annual $100 million amount to burn on 100 players a year, thus creating 100 new millionaire annually. We would have one of the best recruiting classes year in and year out, plus how much we spend would be national news and it would come up in every public interview with every player.