Basically, at their scale it's a hard problem, especially if you need to redo your analysis after every update to an extension and you can't afford a high false positive rate.
Well, Mozilla found a solution. And that's the brute-force solution. Just throw human code-reviewers at the problem until it solves itself.
I'm guessing Google would have more extensions to review than Mozilla and could not rely as much on volunteers as Mozilla can, but Google also has something like an order of magnitude more money, so I'm sure they could work something out, if they really wanted to.
Basically, at their scale it's a hard problem, especially if you need to redo your analysis after every update to an extension and you can't afford a high false positive rate.