There is an opportunity cost for the school if their staff are flying a non school asset while theirs are sitting on the ground. Pretty slim margins on cross-hire, only suitable if the school has no assets of its own which is pretty sad and risky.
Those PA28s and C172s are still around for a reason, they have stood the test of time and the rigours of flying training while many plastic fantastics have come and gone and often taken the flying school down with them.