Not going to imply he stole, but rephrasing the question as why would someone really rich want to steal?

Greed.

It's why politicians who have enough money for several generations continue to take more and more bribes. (I live in Sri Lanka and witness this on a regular blatant basis).

Enough or more than enough isn't a reason for human nature to not want more. It actually seems to be a driver to get more by any means. (my assumption )

Maybe it wasn't about the money and he just wanted to continue working on autonomous vehicles on new terms for a different parent company, without losing all the previous work.

This is also stealing in my opinion, but it might be a little more nuanced than for pure greed.

Greed

Sure. If this individual did actually steal these, it really demonstrates the poisonous nature of greed. I'm not sure how else to explain the failure to quantify the the risk of getting caught which seems to be sufficiently high.