A swivel that spins freely is a nuisance at a dining table. Get up, and the empty chair is left facing the wall; six of them after a meal and the room looks like a waiting area. The chair that turns has to want to come back.
So the bearing is not a plain free-running swivel. A return is built into it: a light spring-loaded detent that holds the chair at the forward-facing position and takes a small, deliberate push to leave. Sit and turn and it moves easily; stand up and let go and it settles back to facing the table.
The bearing itself is set into a recess in the top of the base rather than sandwiched between two plates, so nothing shows in the gap between the seat and the base - and it is fitted with screws from above, which means it can be unscrewed and replaced. A bearing is a mechanical part with a service life; a chair should not be scrapped when it reaches the end of it.
The height of the recess is worked out so the finished seat height is exactly what it would be on a fixed chair. A swivel that adds an inch is a swivel that puts the whole set at the wrong height relative to the table, which is not a trade we would make for a mechanism.