Because it makes people look like pathetic muppets. If you mean "Fuck Joe Biden", say "Fuck Joe Biden", don't clutch your damn pearls about it.
That said, I am perhaps guilty of the same offence there - per the BBC America article "10 British Insults Americans Won't Understand" [1], a muppet is "someone who’s stupid, gullible and incapable of independent thought" - unfortunately the pearl clutching tends to extend to the typical four letter word I would use for such people.
[1]: https://www.bbcamerica.com/blogs/10-british-insults-american...