I have seen questions like this online all the time, "he/she saw the message, how come she/he did not respond?" "I know she/he read the message, but he/she did not respond, does this mean she/he does not like me anymore?" "Why won't my boyfriend/girlfriend respond to my message, is he cheating? He usually does not do this?" "How come she/he won't respond?"
It is a mix of both I think, but more about the world we live in. We expect an answer immediately after we send something because communication is happening at such a fast pace now, but we also have the stigmas that go with responding right after reading. Like "if I respond within a minute or two is that desperate?" or "Does an immediate response mean I have nothing better to do?" questions like that have been associated with responding quickly. On the flip side if they do not respond for a long time we get "do they not like me?" "What else could they be doing?" and so on. It is really rather dumb the games we play with each other and how easily we get ideas in our heads about the worst possible outcome.
I just think it's plain selfishness on their part wanting the other person to answer straight away but not realising people have other things on their minds other than being distracted on the phone all day 24.7...
It totally removes the 'I didn't see it, I was away from my phone, my battery was dead, etc. etc. etc. ' excuse. In a healthy relationship you're able to tell someone you're with that you will get back to them later. As soon as I'm with someone and they turn off read receipts I put them in the "they're just playing around with me" box, and respond accordingly. That is to say I don't trust them and I don't consider the material for A relationship.
My husband and I both have ours turned off but it wouldn't be an issue if it was. He can't check his phone at work and usually gets distracted in the garage so he'll look then not respond because he's busy. I do the same thing where I read something then I'm in the middle of responding when something comes up with work so I stop to deal with it then forget to finish my text. It doesn't bother either of us or our friends. If I had the option with my ex it would've been a very different story. He was very controlling and cheated on me a lot. I kind of always knew but ignored the signs because I didn't have proof. One more warning sign like reading but not responding might have helped me leave sooner. Who knows