I guess that's because of Tesla's advantage in vertical integration -- they control the entire stack unlike Waymo or Uber who have to retrofit their gear into specific car models.
Tesla's tech is generally considered weaker but Musk could still catch up and overtake by playing the "worse is better" card.
They might also realize just how important the combination of electric and self-driving is in terms of $/mi. Both lower the cost substantially, but together things start to get really cheap. When you add in the cost of a driver, internal combustion is still roughly competitive. Once you remove that cost, electric will be significantly cheaper. Tesla's investment into battery production could be difficult for Uber's suppliers to overcome.
The end game in Uber's VC-money-burning present state is an eventual future where self-driving vehicles drive the cost down to where Uber makes a profit from the same fares we pay now. But if Tesla prices their service near their own cost, Uber could get to its own self-driving service and be forced to choose between pricing their service higher than Tesla or continuing to take a loss on each ride.
Tesla also has a huge PR advantage. Competitors need to be able to capture the attention of riders as well as offering self-driving vehicles. Given how easy it is for Musk to get press, he might have the best chance of doing that without having to pay for it in the same way that Uber has had to pay.
Tesla's tech is generally considered weaker but Musk could still catch up and overtake by playing the "worse is better" card.