Because all the c+ communication students and former jocks that become talking heads are rarely also:

Accountants
Attorneys
Business Model Analysts
Tax Experts
Members of Congress

...and ALL those people are the reason the NCAA still exists.

The recent NIL SCOTUS ruling has zero financial affect on the NCAA. Member institutions still pay their fees. The NCAA still has considerable authority, be it as I have long argued, limited because it's association bylaws do not trump state legislatures or congress. If anything, the new NIL has simply made it much tougher for universities to raise contributions from donors and their business institutions. It's not like the cash pie got bigger with NIL. It's not like businesses that previously spent 6% of revenue on charitable or marketing promotions tied to a university athletic department is suddenly going to pump that up to 7% or more. The pie is in this case, just getting sliced into a lot more pieces. As for why the NCAA will exist:

NCAA member institutions are:

1. Competing as quasi professional entertainment sports franchises against one another and
2. That makes them businesses.
3. As businesses they are exempt from a ton of special conditions and policies, laws and regulations that PROFESSIONAL SPORTS ENTERTAINMENT LEAGUES (NBA, MLS, MLB, NFL, NHL, etc.) must all comply thereto.

THEREFORE: THE Biggest reason they stay in the NCAA is that if they leave:

4. they become direct business competitors to those pro sports leagues.
5. Although they already are, the NCAA shields them and their revenue from a lot of the competitive equalizers.

When Michigan sells a baseball cap, the funds go to the University and disappear. When the Detroit Lion sell a cap, the money goes through the state and federal tax corporate income tax filter. Trust me, the Lions, Red Wings, Pistons, Tigers (and the like everywhere) across the board see the competition is getting special perks, it will balk. Professional leagues are NOT going to just sit back and let UM, MSU, etc. compete with them for concession dollars, ticket revenue and TV revenue on a non-equal tax basis.

The reason the P5 leagues have NOT bolted the NCAA is because it protects them from professional sports business realities.