For example, it's dead easy to make a high-capacity message queue by just using SELECT ... FOR UPDATE SKIP LOCKED with Postgres transactions, and I would argue it's more reliable than a lot of microservice-everything setups by way of having very few moving parts.
Classic NIH syndrome. "I made it myself so it must be better", when it's clear that a single sql query doesn't remotely approach a complete solution for scheduling. But the ignorant use it because they don't know better, until they too fall into the trap and realize they spent 10x as much engineering work to get something they could have just installed from the web and been done with. Every generation seems to fall into this trap with another tech stack.
It's all trade offs, right? Introducing a new component to your stack isn't free, it's paid for by more operational complexity. Maybe it's worth it, maybe it's not, but there is a calculation that needs to be made that's not just "NIH syndrome".
If you already have a DB (and essentially every app does), it can be far less effort with the same or greater reliability to create a queue table than to set up RabbitMQ, NATS, etc. As long as you tune the vacuuming appropriately, it will last for quite a lot of scale.
Source: am a DBRE, and have ran self-hosted RabbitMQ and NATS clusters.