@welder puts it very nicely.
While PTFE may be very 'slippery', your swivel is landing on silt, clay, gravel, or sand or whatever else is on the lakebed, any of those are enough to 'jam' the swivel and stop it rotating.
Add to that, your rig is in a direct line from the mainline, be that straight out on a pendant or inline set-up, or at 90% on a helicopter set-up.
The swivel is basically a simple way to attach your rig to the mainline. It doesn't really need to swivel at all, unless on a helicopter or Chod set-up when it may need to spin during the fight.