I agree many people go off topic. Some of it is as neko explained I think (evolving discussions) that are just "walking away" from where the original poster probably wanted to go with the discussion. Since this is a public forum, that happens.
Some of it may be trolls trolling, snarks snarking or even emos emoting.

but it happens, and it probably will not change and it happens only to the extent that it is allowed to, no more.
Either way, i would not let it "urk" you to death. maybe "something that I was just thinking that's annoying" but urk to death?
Do I disagree it can be frustrating? Nope.
Do I think it will stop? nope.
Do i think it is something we can live with? yeah.
I hope this helps somehow.