That's a great question. The answer to "why wouldn't you use Mercury instead?" should hopefully be more clear when you watch the meta-interpeter video. The syntax of Prolog is not accidental, and this is a case where it really matters. You sacrifice a lot of power by introducing things like dot access notation.

Additionally, there is nothing stopping you from writing pure monotonic Prolog, which does not involve cuts and uses pure IO.

Circling back to DCGs, the combination of DCGs plus interpreters, which can be expressed in a handful of lines of code, allows for powerful language crafting.

While constraint solvers are not unique to Prolog, CS+DCG+MI is a killer combination I've only ever seen in Prolog that when combined allow you to solve some wicked problems rather elegantly.

That being said, thanks for the reminder about Mercury, that looks like it could come in handy on some Java-only projects.

This is DCG+CS only, still quite powerful and I have adapted it for GOAP and Hierarchical Task Networks quite successfully: https://www.metalevel.at/zurg/

Finally, I will agree with you that there is a LOT of jank Prolog, but not ALL Prolog is jank, and some Prolog is truly exquisite-- I hope you will keep an eye out for it.