Ddd I must admit I'm someone who does this
I always see it because of the notification but why I usually do it is I'll go to see the post and the reply is:
1.) Something I don't care to give a response to
2.) Something along the lines of "Goodbye" so the chat is over
3.) Something that's too long and I want to think about before I reply
4.) Something that's dumb or irrelevant and doesn't need a reply
5.) Something that is sparking a conversation I don't want to have
Or I'll either be tired or busy, look at it real quick, leave the page and then forget to come back to it (which is usually the most common one.)