My nephew got a iPad glass replacement by a “professional” repair company.. the glass shifts around (because they didn’t use enough glue) and the audio port no longer works. You get what you pay for. With Apple you get your device back in mint condition.
Because often the repair involves putting the motherboard in a completely new outer shell which is extremely difficult to get hold of unless you're Apple and has to be glued together. Hence it is not repairable.