Comment by everforward

The data tier is one of the lowest tiers in the stack (for bigcorps with centralized DBs), and changes bubble up the stack. Change an API and a few upstreams have to change. Change the core DB and _everything_ has to change.

That makes the schema calcify, and the DB becomes the most stable format of the data. I've worked more than one place where "upgrade the core DB between major versions" was a multi-quarter effort.