Part of the problem may be lack of tagging and filtering tools.
For example:
I'm not interested in memes so if a community is largely filled with them, my only way to avoid them may be to block the community. This includes communities that I might otherwise subscribe to, or want to engage with.
This is also tied into community fragmentation, community discoverability, and feeling the need to browse All to see anything. I don't know how widespread my issue is, but I have seen others mentioning having extensive block lists of communities.