no one else has to look at it
That's only partially true. Certainly we can ignore the thread in its entirety. But if the thread contains something that is interesting as well as a whole lot of "wind and fury" it does take some effort to disentangle them. Even if I ignore by certain people in the thread, it messes up the paging and such like by making so many post irrelevant. And since each new post resets where the blocks of 50 are counted from, it can still be an irritation that gobbles up my data allowance on the phone for no value to me.