Well because the mass amount of data that would be grabbed in the event of an accident would far overshadow a reasonable amount of capture memory during normal driving, which would utilize a lesser set of sensors and maybe lower grade video, which didn't have to factor into the explanation for the accident.
256GB of flash is just over $100 right now. Storage is not a problem. Even AIRCRAFT do not have a problem with storage and they have a LOT more data to store.
Step 2 would include choices such as hit the breaks if it would work. I just used summary steps to make it easy to understand.
Taking power from the engine is NOT the same a braking.
Taking your foot off the gas is NOT the same as stepping on the brake.
Seriously. Try it on a hill. You might end up going FASTER at the bottom of the hill than at the top.
Your plates store information about your car, hence you know from looking the number up, everything to know about the car via reference lookup.
Make/model/year/VIN/owner/owner's address. And maybe whether it passed inspection or not.
How will knowing the VIN tell you anything about hitting it?
Or the owner's address?
Or the owner's name?
Or any of the other information?
And what happens when the site you're trying to use to look up that useless information is slow?