That's the fallacy. That keeping it all in one place with one tool is somehow better, because you can, or because it's all tightly coupled, or because it's one less piece of software. But in practice it just creates more problems (lots of them) that a dedicated, separate queue does not have.

If you want a very simple way of looking at it, think about how code becomes modular. You don't put all your code in one function, or one class, or one file, or even one application. Not just because it's easier to read, but because specific components should have clearly defined delineations from others, and forceably be kept separate at the borders of their functionality. Doing that leads to benefits that outweigh them sitting next to each other, tightly tied into each other.

At the very least, you should not be using one giant database for all your data in all your applications. Everybody should know that by now. And just like you shouldn't conflate different applications' data containers/interfaces/internals, you shouldn't conflate a queueing system with a completely separate data container. They need to be separate to avoid a bunch of problems. Again, not because you can't do it - like keeping all your code in a single function, or class - but because it's going to lead to a bad time.