Because you could, you could argue you forgot. It's a way to get around it. It's easier if apple controls who fixes phones.

It's easier to block the part checking for individual parts than the entire os. You would still have people with bricked phones once they got home and the faulty part phones home.

If apple didn't do this their phones would be stolen at a very high rate. Especially from tourists. The phone would also get a reputation for being stolen. Or stopped working once you got home from the repair store.

Apple sells the convince of a device that works reliably. This makes it of very high value. Especially to those that don't want to worry about their tech. So apples repair methods keep this value. If they changed their phones would be less valuable to most consumers. If this wasn't the case then fair phone would outsell iPhones.

The phones are still repairable, but it's restricted to apple and those apple have authorised to do it. The solution is to make repairs cheaper, not make interchangeable parts. It doesn't work on the scale apple operates, for devices of significant value that can be pick pocketed and shipped to another country.