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.
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)
a lot of younger folks are impatient (past me included) and get suboptimal results for a lot of time spent, which puts them off of doing work themselves. the irony is of course that younger folks have all the time and none of the money. they are also the ones complaining the most about how expensive home/auto/whatever services are.
this will also teach you the limits of what you actually can and can't do, so that when you do spend money on a pro it's money well spent. they also prefer to do the more complex jobs (less customer overhead, setup and cleanup time eating into margins) so it's win-win. as someone above said, they're far more efficient at the actual work but you can't make a lift go faster or shortcut any consumables used.