If they're using external speakers, the only reason you're not hearing echo is because it's being software-cancelled. Different systems are better or worse at this software-cancelling; phones are good, Apple computers are good, otherwise YMMV.
Yet one sees other people utterly convinced that using external speakers is bad, bad, bad.
That may explain it.
Some people have a hard time realizing that they're interrupting someone else because that other person's audio is getting ducked while the laptop prioritizes mic input over speaker output - with the intent to reduce echo.
Basically, of all the ostensibly unmuted mics, only the one with the loudest human is truly unmuted.
It's closer to half-duplex than full-duplex. Full-duplex with no artifacts requires no echo cancellation which requires headphones.
It's easy to have conversations with friends on discord where 3-4 people are talking at once all with headphones. However this has never worked on a zoom or hangout with less techy family members or work colleagues using ext. speakers.