For the uninitiated 3DoF means the headset only tracks the rotation of your head, not your heads absolute position as you move around, while 6DoF tracking does both. 6DoF is also much harder to implement.
Probably some have tried and I’ll be curious to know what prevent it.
A further complication is that the IMUs you find in phones, especially back when all this was new, weren't very good. So even a plain 3DoF experience would suffer from slow response and yaw-drift from lack of magnetometer, which is why GearVR had an extra IMU in the headset and why Daydream required new certified phones with better IMUs.
3DoF can still be good enough for VR180/360 movies, which don't allow movement to begin with. But as far as gaming is concerned, you really do need 6DoF, as even a bit of leaning forward in a chair will make it instantly obvious that the headset motion and the headset visuals are out of sync.