To somewhat paraphrase my other comment in the thread: Most people on the Internet aren't writing personal letters, and most writing on the Internet isn't deeply personal writing from someone you know and have ane motional bond with. Deeply personal writing isn't something easily comparable to emoji, either.

Fair, but handwritten letters were much longer (because that's what made sense when the round trip time was measured in days/weeks). With multiple paragraphs you can build up a rich emotional context for whatever you're saying.