There's just some distaste about people agreeing to be in the military, getting 4 years of education and then bailing on that commitment.

Deferment addresses that on the surface, but really only works if the guy relatively quickly fails in the NFL.

The bigger issue is with the possible doors it opens up. What if someone gets WNBA drafted? Or offered a contract with the German basketball league? Is being a UDFA enough to defer commitment? What if I'm an excellent archer and have other opportunities? Or coding? Can I take two years to see if I fit at Google?

That's the real reason. Because nuance is hard, especially in a huge bureaucratic organization