Reality of a Back Glass Repair

Hello, does anyone know why it is so expensive and convoluted to get the back glass repaired? I recently did it myself and it cost about $485 and was a terrible experience. Not only did they not give me aheads up that they were about to replace all of the interior parts as well, they also threw out my sim card as well that was programmed to my cell carrier. None of which I was informed about. It did not stop there either. We ended up going back later on after an appointment I had because the woman had told me that I needed to set up my phone afterwards. To many consumers, especially those unaware, set up usually means sign in and maybe get your data off of icloud and then continue. I was not informed that I had to re-register my device with verizon (my carrier) because they had thrown away my other parts. As an engineer I would never tell someone that they needed to replace an entire router because one of the ports was broken after all. So my question to the group is why is it so expensive to fix a very basic part when it is not in fact welded to the internal components. Knocking off the glass and gluing a new one on would be more time consuming but would end up saving a client money and time, but instead I now have a headache and was admonished by an employee for not being aware of the situation when I went back. Thanks for the information and I hope someone else finds this information insightful if they end up with the same situation.