I maintain many open source projects on GitHub and elsewhere (over 160 as of this writing). I have merged and/or closed thousands of Pull Requests (PRs) and patches in the past few years, and would like to summarize here many of the reasons I don't merge many PRs.
A few of my projects have co-maintainers, but most are just me. The bus factor is low, but I offset that by granting very open licenses and encouraging forks. I also devote a set amount of time (averaging 5-10 hours/week) to my OSS project maintenance, and have a personal budget of around $1,000/year to devote to infrastructure to support my projects (that's more than most for-profit companies who use my projects devote to OSS, sadly).
I don't like closing a PR without merging, because a PR means someone liked my project enough to contribute back. But sometimes it's necessary. I'm not trying to be a jerk (and I usually start by thanking the contributor to try to soften the blow of a closed PR), I'm just ensuring the continued health of the project. Below are the principles behind how I maintain my projects, and hopefully by reading through them you'll understand more about why I choose to close PRs instead of merging.