The succinct reason is you're dependent upon them. Over the years I've invested time in learning and implementing their API only to see them get acquired by a

The succinct reason is you're dependent upon them. Over the years I've invested time in learning and implementing their API only to see them get acquired by a competitor that told me I had to use theirs. And early on I had one spamming with my domain names. And I had one shut down without notice. Nowadays they can be expensive. You can spin up a $12 a month DO server that will handle quite a bit of mail. For me, MAIB having a DNS server built-in was a big plus too. I have several domain names and email being sent by them or just email addresses I used for others to contact me, and alias addresses that all go to one email box, and some of my domain names are just for fun. For those I can create a quick email alias so it sends email to an existing account. I use my DNS server IP address first on the list of dns servers in my network preferences on my Mac so when I make any changes to the sites I'm working on they show up instantly. I don't have to wait for them to propagate to find out I broke something. So, it's really more than just email, but having that completely under my control has been a good thing. A pita sometimes, but still a good thing.