Is it considered that hard and expensive to just run a full backend stack on your own machines, be it on dedicated servers or AWS etc.? I guess it might even be cheaper to have someone do a professional server setup for you instead of having to deal with all those service providers.
Well time is money. Things that are not core to your business like managing email servers are better off outsourced; not because they are hard, but because it frees your time from setting up, managing, securing and monitoring your email servers, and allows you to put it in something that actually matters to what you do.
I do agree that eventually everything should be self-managed to ensure service is never disrupted. It's critical to ensure that the service is always online for the customer and maintaining that control, in my opinion, is important.
I could push to a self-owned git repo, but bitbucket and github have lots of nice tools already built around them. When they have issues with their servers/disks, most of the time I won't know and don't have to do anything.
Now, by all means, have contingency plans. Keep pushing to that self-owned git repo, have back ups and such, but I think people just don't want to worry about things.
Furthermore, a lot of these free (gratis) applications can be replaced by free (libre) applications. That way you actually can run the software on your own machines.