I've asked them why not, and the consensus seems to be that not putting a question mark at the end of a question makes it sarcastic or a rhetorical question.
I have no idea where they got this from and it is totally wrong.
The thing is, most sentences are imparting something from the poster to the reader. Stating facts, opinions, etc.
The thing about questions is they are requiring something from the reader rather than telling them something. They ask instead.
So, it's a completely different type of sentence and needs its marker for that. I'm sure my eyes skip ahead to see what kind of sentence I'm reading. A question mark is a clear communication of what type of communication it is.
Those who don't use them when they should are either illiterate or ESLs with no respect for the English language. It is an affront to read these posts.
It makes it hard to easily understand the poster.
Why would someone make a post but yet want to be misunderstood?