>>42378453 (OP)Because it would work like a prosthetic, and like a prosthetic, it would have to be continually updated as she got older.
That's saying nothing of the potential cardiovascular issues that could be caused by altering a part of her body that was underdeveloped and that the body had compensated for during her early foalhood.
Finally, if she were to encounter anything with anti-magical properties, the spell would break and lead to serious complications. Imagine you're just flying along, but you pass through some anti-magic field, and suddenly all the tissue and bone mass associated with your wings is going through rapid cell death.