Any regular student can transfer anytime he or she wants too. Why shouldn't student-athletes be able to do the same?
I posted some time ago but there was a comparison done between transfer rates of student athletes vs regular students. When players transfer, they are not transferring for academic reasons. They tend to end up in a situation where they transfer and have difficulty landing in an academic program that fits what they were in previously. Credits are lost because schools are often offering "boutique" classes that other universities won't recognize (though they offer their own). Often universities have to create a program/major to accommodate them. The general outcome is delayed academic progress and failure to graduate. Whereas for regular students, they are generally transferring BECAUSE they want to graduate and get on with life.
The system is such that players can be enticed to transfer for more money as
@bayrooster correctly notes, even though it is almost never in their academic best interest. It makes sense that the grown ups would put some safeguards in place. There used to be a lot more safeguards and restrictions in place to do what they could to get players graduated, but a lot of that has been stripped away.
Theoretically should they be allowed to transfer as freely as other students? Probably. Though universities invest A LOT more in student athletes than they do regular students, so I think the schools deserve some protections as well. In practice, though, the restrictions are good to protect, to some degree, the student aspect of the student athlete.