I also doubt the bore enjoys boring people

I'm sure they like to think they're entertaining. Which, ironically, incentivises their investing very little in finding out they actually are. I honestly would expect someone to react pretty badly to being told or hinted at that they're abusing your patience - and I think that's why people get pissed off with such people. There's little option to exit the conversation properly.

IME, bores tend to just find themselves with fewer and fewer friends as time goes on - often without ever finding out why. Because how can you tell them? Costs you social points for dubious gain.