Just not that straightforward in practice. You have all of these product lines that people are building that you're hoping will grow the business. They all depend on your backend stuff that's just an implementation detail. You have to somehow convince everyone across the org to stall their product development to perform a "migrate to Postgres" thing? It's not going to be easy.

There was a recent big company that posted on Twitter about "shutting down our last Oracle server" and that was the last thing in a multi-year process or something like that.

Coordination is sometimes harder than the technology itself.

Because vendors are not fungible in the eyes of the buyer.

Lack of capability, mismanagement, misinformation to name a few