Even something as simple as an oil change can go wrong. Drain plug broke and you need a new one? Or you forgot to put a gasket and it leaks and now all your oil is on the floor? Well, now you need a trip to the parts store, or order it and wait until parts get there.
In my case, it was a routine maintenance operation: changing brake fluid. I used a power bleeder, and at a third of the pressure specified on the manual, the reservoir ruptured. So now I needed a new reservoir. Parts store didn't have it, and it took 2 weeks to get there.
Hey, better this happen while you're wrenching than when you're zooming through the twisties and need to stop
Luckily that one was just a quick trip to the parts store to get the right fluid and reconnecting the line properly this time!
Often I'd invite a friend over while I did the work, keeps me company and that way they can be the backup vehicle (and I'd return the favour).
This is the other reason a lot of people will do their own motorbike maintenance - for a lot of people, the motorbike is the second vehicle.
It's also rare to order too many parts or tools. Rental tools are very useful, but many times as a car ages, you may need to order other parts.
Though I don't get this complaint; because there is often lead time at a mechanic's for the parts too; you still need a backup transportation plan for that situation. (And it's even more likely to effect you during a 9-5 if you your wrenching on weekends or weeknights)
Because just like we have bugs in production, things break when you wrench on them, even if you do everything by the book (note: first-hand experience there).
But those are just setbacks, and no big deal if you are prepared for them.