There are simply too many people on Mastodon who got used to their cosy little underwhelming microblogging project without even realising how underwhelming it is. The revelation that the Fediverse is more than Mastodon, and people can post to their timelines from much more powerful projects is deeply disturbing to them.
First they rub their eyes in disbelief when they come across their first "toot" with over 500 characters. Then they feel disturbed because they think that there must be some evil hack involved. And when you tell them that there's more to the Fediverse than their beloved vanilla Mastodon, they want the Fediverse to turn back into something it has never been.
And yes, some actually demand content warnings for long posts, although I've yet to find out where "long" starts. Probably at over 500 characters. I've actually supposed that demanding long posts for text formatting will be the next thing. In fact, I'm pretty sure there are people who block everyone who uses either.
Not to mention that our likes probably can't get away with supporting our way of content warnings, namely with keywords or hashtags. Sure, that'd help Mastodon users who use filters as well, but the majority of Mastodon users probably doesn't even know that Mastodon has filters because their app doesn't have controls for the filters, and they've never in their lives seen Mastodon's Web interface.