The theory I’ve heard is that people on reality TV shows would do this so the mics could pick up their conversations better. So naturally, brainless idiots without an original thought in their dense godforsaken skulls people who watch those shows started doing it in real life too because they saw popular people doing it on TV.
It’s just a theory, but it seems plausible because it’s clearly not how phones were designed to function on speakerphone or otherwise.