Mostly because the tooling is so bad. Or do you have unit tests, lint, and easy version control for your stored procedures? Are they written in a way that matches at all what rest of your programming is? Can you import a randomly picked utility library?
The next obvious question is, why is tooling so bad then? And would it have been so bad if we invested as much into RDBMS as we did into Node.js web-frameworks-of-the-day.
Writing the next modern elegant artisanal javascript webshit is a lot easier than making a sandboxed programming environment that integrates tightly with a production quality database engine and has a good story for testing, deployment, debugging, etc.
