You're assuming that I'm just talking about hosting costs. I'm not.
Maintaining VPS infrastructure just to run a DB is a huge amount of overhead. More so, when the DB crashes and/or wildly consumes memory. You need disk/memory monitoring, logging, process monitoring, availability monitoring etc. What if the VPS itself goes down? What automated system is going to spin it back up?
A self-managed VPS DIY solution is substantially more expensive to maintain than using well-established tooling like k8s. These are all already solved problems. Not just that, it's a standardised work-flow with the rest of our infrastructure i.e. no manual futzing around with VPC etc. to get our cluster and externally managed VPS talking to each other.
For us, a DIY solution was never even on the cards. It was either a fully managed service or what we have now.
However, fully managed services don't just cost more per month. There's frequently additional development overhead, as the offerings are "off the shelf" software and not necessarily easily customised. For example, right now our PSQL deployment is running a custom PSQL extension that seamlessly handles BSON (MongoDB) Object IDs, semantically converting them to UUIDs. Thus, post-migration from MongoDB, application code isn't riddled with code handling legacy IDs.
Monthly service provider fees are but one consideration when performing costings.
EDIT: It's probably also worth noting that our entire infrastructure, including DBs, can be spun up as a k8s cluster on a developer's work machine in one command. We of course also have a staging environment, which mimics production. Development also ought to mimic production as closely as possible (but still be debuggable).