They also don't want to alienate their customers. Imagine if you bought a new, $100,000 Mercedes, then they announced a week later that the new one would be more powerful, better styled, had all sorts of new gadgets, and would be immediately on sale for $100,000.
You'd be pretty pissed. You're spending major money, and would want to know that you're spending it correctly. That's part of why the big manufacturers have year-long hype and PR cycles. You don't want to surprise your customers.
The other problem with quickly making new models is repairs. Tesla actually has a lot of luck in this space, because mechanically, their cars are a LOT simpler than a normal, gas powered car. They don't have to worry about things like gearboxes or clutches. However, it is a tendentious benefit for the consumer, especially as the cars become more common, to have spare parts easily ordered or at hand.
Well, yeah, but I bet a big part of that is how we haven't adapted to expect that to happen, the way we do for computers, smartphones, TVs, etc.
And the rest of it is probably attributable to cars being a much larger, less often-per-person purchase than a computer or smartphone.
Yes! Yes! Yes! As someone who has owned 30yo motorcycles that went out of production 30 years ago, and also 30yo motorcycles that are still in production, the difference is tremendous.
Vehicles that have been around for a while have superb parts availability, and a community often develops with a large amount of very specific know-how. Those of you who have not wrenched on vehicles may not realize this, but sometimes the community can be even more valuable than any other feature of the vehicle.