Mostly because databases are the key piece o data-persistence infrastructure. Spinning up a MySQL db to dev against, or a single server for a hobby project is quick and easy.

In production, all of a sudden you have a lot of work to do, especially around HA. Figure out replication, get it working, figure out how to monitor/alert if it stops working, figure out failover, figure out how to test that failover actually works, etc.

Support around that stuff has improved over the years, but it's still non-trivial and high-risk to DIY. It's a very different scenario than a stateless app server where you can have easy redundancy.