The problem is that a lot of what people post falls into more of a gray area than legally bulletproof death threat. This is the problem with dog-whistling[0] generally, it's very easy to make statements that you can later deny the intent of, which makes them hard to prosecute.

For example, how would law enforcement go after someone who repeatedly said something like "Someone needs to take care of John Q. Politician because he's ruining the country"? That's just not legally sound enough to preemptively arrest the poster of that statement, but so many people talk that way online.

0: https://en.wikipedia.org/wiki/Dog_whistle_(politics)