They can certainly do it, but I suspect that they simply don’t want the ‘hassle’ that comes with partial repairs.
Years ago (20+), one of my previous employers also had repairs made to electrical equipment, you don’t know how thankless work that can sometimes be, with ‘customers’ sometimes coming back months later with ‘yes, this device has been here for six months. repaired ago, now the device is defective again (often in a completely unrelated place or function), yes, you now have to reimburse this. Amazing sometimes.
I therefore understand very well that parties such as Apple, even though you might sometimes prefer to see it differently, simply replace the entire motherboard for repairs that are not covered by warranty (or, for example, the top case for keyboard-related matters) and then this defective motherboard, for example. refurbish again for the next repair. This will also generate more turnover for them, in addition to the benefits of having to argue less with customers who are trying to cheat you.
The customer can then opt for repairs at Apple with complete replacement, but also have partial repairs carried out at companies such as Louis Rossman. These companies also have their right to exist.