I've been participating in online discussions for a long time, but I'm still not able to understand people that receive constructive replies to their posts and just leave them on read...

I'm unable to do that myself. I feel I owe the person that took time of their day to say something to me to acknowledge their reply somehow. Either as a like, or a "thanx", or anything but silence...

Saying something constructive to someone and receiving no reaction back from them is quite unpleasant to me. :)

PS. This is not targeted any anyone here. This happens a lot in other fora.