Granted it's still very much a transitional period, but that just might not be possible with college players at a certain level going forward. Whereas the draft gives you exclusive negotiating rights over another 29 teams, now you've got to compete with, potentially, multiple college programs who might be prepared to raise their offer in the portal and can also offer the kid a better playing lifestyle (as well as education) for one more year. That's harder to manage than just a binary choice between staying in the school he's already at or going pro.
And on top of that, you kind of are competing with other MLB teams indirectly. Forget Trop, it's possible Spencer just got the money that might have otherwise gone to UT trying to keep the Mets' 14th-round pick, Tanner Witt, and now maybe the Mets will have better luck signing him. Though I'm sure if the Horns want all of these arms they'll have the money.
The possibility of Spencer transferring to UT also probably didn't even exist when the Phillies completed most of their scouting on him because UT hadn't fired its coaches yet.