Tesla only uses cameras for it's self-driving. [1] This is theoretically possible but actually really difficult to implement in a way where the car "understands" all situations.

Waymo uses cameras, radar and lidar for its self-driving. I think they also use GPS and super-detailed maps. This makes it much easier for Waymo to track the road (from maps) and know the distance to obstacles even if it does not know what the obstacle is.

Basically, Tesla is trying to make a self-driving car that uses only the information available to a human which means their AI has to be way "smarter" than Waymo's.

[1] I think Tesla is adding radar, but I'm not sure about that.