They certainly could do it, but I suspect they simply don’t want the “hassle” that comes with partial repairs.
Years ago (over 20 years), one of my previous employers also carried out repairs on electrical equipment, you don’t know how thankless work that can be sometimes, with ‘customers’ sometimes coming back months later saying ‘yes, this one was done “Here for six months, repaired before that, and now the device is faulty again (often in a completely unrelated location or function), yes, you now have to pay that amount. Amazing sometimes.

So I understand very well that parties like Apple, although you might sometimes prefer to see it differently, just replace the entire motherboard for repairs not covered by warranty (or, for example, the upper case for keyboard-related issues) and then those are refurbished. A faulty motherboard, for example, is returned for the next repair. This will also generate more sales for them, as well as the benefits of having to argue less with customers who try to scam you.

The customer can then opt for repairs at Apple with full replacement, but partial repairs can also be done at companies like Louis Rossman. These companies also have a right to exist.