How so, exactly? There's no way for anyone to check how much a battery was charged regularly, just like there's no way to check if a car was only used to drive to church once a week or driven like a hooligan hitting redline all the time - I'm not suggesting fraud of course, far from it - just that buyers have to assume the worst possible use case, and if so I don't see how actually regularly charging the battery to max affects the resale value at all. Besides, I don't expect the resale value to be very high after 8 years(realistically more) anyway.
I believe the SoC (state of charge) history is recorded for Teslas and other EVs. There is a company called Recurrent [1] which uses this data to estimate battery degradation. I think the idea is to generate something like a Carfax report for used EV battery packs.
I agree this is a cultural problem. Houses are seen as retirement vehicle, etc. I think it's partially a sales tactic, to switch your brain into "investing" mode and willing to fork up a lot of cash and leverage (I'm "investing" in a Tesla, see how much gas i'll save?).
It's double edged, the first edge is selfish, maximum extraction of value. The second is definitely not exclusive to Americans - taking care of your belongings and reducing the rate of sending things to landfill.
Because the better the resale, the nicer the car you can afford to replace it. Most new cars are kept for 3 years and traded in, (or leased) so the better the resale the more money to put into your next car. Or if you lease the less your lease will be.
Those who buy used cars keep their cars longer, and are willing to pay more for a car they think will last longer.