You switch to a third party mail provider for the same reason you buy a new car instead of driving around your classic you've been fixing up as a daily driver. Not because you don't like it, but because sometimes there's benefit in offloading the annoying bits of your hobby so you can focus on the interesting bits.

Fixing up an old car is fun. Doing it in chunks of a few hours so you can drive it work the next day is less fun.

Running your own mail server is a fun challenge at first, as you have to figure out why some random site won't talk to your server, or why your load spikes at random times. Then you decide that diagnosing other people's broken configs and dealing with drive by spammers is annoying and boring.