The problem is ensuring deployment matches up. It's very easy to end up with subtle differences between a newly deployed database instance and a database instance that was deployed with an old version and then updated. For code you would think very hard before deploying each version as a patch to the previous version - it's easy and effective to just deploy the code afresh each time, with a complete artifact built from a specific VCS tag. It's much harder to do that with databases; the tooling just isn't there and it's hard to ensure that you wipe out previous logic but retain previous data, because data and logic are commingled. You could possibly build a system for this kind of thing, but the standardized tooling just isn't there.