Everyone hates sending back a firearm for warranty work about as bad as they hate going to the doctor for a prostate exam.
I would imagine there are more than a few reasons for this, such as:
1. As much as I hate to say this, there are some less than honest owners that may try to pull a scam, by sending the unserialized slide from another similar firearm for repair, or actually causing the problem through carelessness or abuse but trying to get the firearm fixed for free by the manufacturer.
2. The manufacturer wants to make sure that the repair work has been done correctly. If the home repair was botched by the owner then the manufacturer could be stuck with repairing more than originally claimed, even if it was the owner's fault.
3. If the repair was done incorrectly, the firearm might fail again resulting in a second warranty claim, or even worse, injury to the owner.
4. There are probably some legal requirements that the work be done by a certified gunsmith so that the manufacturer is not held liable for shoddy workmanship. We're living in a society that loves to sue companies. Making sure the firearm is properly repaired and having a record of that repair helps protect the manufacturer from possible lawsuits. It also benefits the owner too.
5. Most manufacturers require that the firearm be returned for repair. In order for the warranty to stay in effect, we have to follow rules.