Well, we don’t know for sure that they’re not doing that, or at least Trying to, but more than that, it’s quite possible and even likely that doing that would not meet the testing requirements. If I were adding beta testers I would want a testing cohort that covered as much of the country as possible and as many driving conditions (city, suburbs, country, etc) as possible. If they have 5000 people trying to get in to FSD beta in Palo Alto but no one participating in Fargo I would expect the person who signs up in Fargo to get in sooner, just because they need the data.
Another potential factor is equipment. We know Tesla has updated the equipment in the cars. It may be that newer cars are better equipped to run the beta software. (This is just a guess, I have no idea if it’s true or not.)
’All things being equal’ I don’t think there’s anything wrong with your idea but I suspect all things are not actually equal.