Because there are many times where transfers help kids. It's not always on the field, but it could be a social issue or an educational issue.
But keeping it on the football field, outlawing transfers is locking a kid in when coaches have freedom to move around. If you're a WR and your new coach runs the double wing, you should have the right to change schools for example. That's how we got the transfer portal in the first place in college football.
Not to mention you would have lawsuits out the a** from parents not being able to have a say so in where their kid plays sports. Horrible idea