We have text messages, voicemail, Whatsapp, Messenger, and PM systems on various SM platforms. Not to mention 'phone calls on both mobiles and landline (I still have mine).
... all demanding our attention during various points in the day. It's too much IMO - an overload.
I have told all my friends and family that - unless it's an urgent matter - I will respond when I have the time and they understand.
Sometimes, it's also having the inclination. I think we've all had messages and mails that are 'chatty' in nature where the things that are said would be acknowledged with a nod or a smile in physical conversation, but for which we have to find suitable words to 'appreciate' in a message. For me, late night is when I have the time to focus on such messages, and I'm often too fatigued for it. In reply to long messages which have outlined the minutiae of a friend's day (or week!)... I simply want to message back, "nice to hear from you" but feel compelled to reply to each bit of 'chat'. And that is often tedious.