I'm not saying you are wrong, but that seems weird to me.
Every university in the country probably has hundreds of millions in endowments, but obviously those are related to running the academic side of those universities. Not sure why anybody would have an issue with starting one specific to sports. Having typed that, I'd guess some already exist. Over-simplified, but all an endowment really is is a permanently restricted donor contribution. Surely somebody has donated to athletics over our history and said hey, don't touch the corpus, but you can spend the earnings on OU athletics.
If there is any push back, I'd guess it's that OU needs the funds now. Short-sighted, but that could simply mean we haven't gotten that contribution large enough to stick it away for the future.