Why Does Unity call Unity.Physics.Systems.BuildPhysicsWorld multiple times per frame?

[buoyant moat]

Is it because BuildPhysicsWorld is happening on some kind of Fixed Time Step?

[rare talon]

Exactly as you said. It builds multiple times per frame because it's run in a FixedStep

By default it updates at 60fps so if you have less than 60fps then it needs to update multiple times a frame