Four obvious reasons:

1. They've been doing it for ages. They had cars on the street fifteen years ago.

2. They bet on hardware that's not just cameras. Cameras—in practice—are still not the best tool for the job. Cameras see in 2D, they get dirty, they are easily blinded and obscured by dirt, etc.

3. They have data from every Google Street view and mapping car ever deployed. They have the most data and the most current data. Every Tesla on the road would need to be maxing out its LTE connection all the time and they still wouldn't have the breath and quality of data that Google has.

4. Google is throwing money at Waymo. They can see the potential profit if they win. They're not going to get dumped like Cruise.