In my experience, most people will choose to listen to music directly from a phone or laptop even when there is a good floor speaker system in the same room they could connect to and play through in seconds, because they just don't care.
If you're talking about someone hosting a party where they're playing music off their phone/laptop, that's something I've never personally seen before lol
I think you hit the nail on the head- people think a big stereo is for volume levels, not quality. They will obviously switch to it when they want things louder, but often not before.
When I went to public high school in rural America, pretty much everyone I knew owned some sort of headphone/speaker system, and they would be used at every party / larger gathering where audio was important.
It depends on the context though. Obviously if someone is just sharing something casual like a Youtube video on their phone, they're going to use the built-in speakers.