All the above, particularly @jdman’s comment. The leg muscles are expense energy-wise while offering limited propulsion vis a vis the upper body. There are some triathletes who are big kickers in distance events, but they are rare and 100% bring that over from their swimming background. In most cases, nearly all, that kick gets shut down and limited to a) helping with rotation, b) quick accelerations (such as starting, beach entries, leaving or joining a pack, getting around a buoy, etc), c) warn someone swimming too close to you, d) or to start getting blood flowing to the legs as you approach the exit, whether ramp or beach which requires kicking for catching and managing the waves to speed your exit). Open-water conditions, such as high waves or high frequency of waves, may mean (a) - helping with rotation - requires more kicking than in a typical lake swim.
If you find your legs are sinking when you’re not using the buoy, then you’re probably kicking to keep your legs up. In other words, you’re compensating. Practice pushing your chest into the water, make sure you’re looking down and not forward, and work on, if you’re not doing it already, a good extension with your stroke that encourages you to rotate your shoulders for a greater stretch. Practice “front quadrant” swimming (though I think that’s now an antiquated, disused term) where one hand doesn’t start the pull until the other is at or past your ear. That’ll help you push your front into the water.