it seems prudent that manufacturers try to ensure that headlights stay in good shape for longer.
It would seem so, but until there are toothy laws to ensure it, it won't happen. Some states have inspection programs and should reject cars based on yellowed/crazed/hazed exterior lenses, but seldom do. And to establish standards of reflector performance after X years, and for state inspections to inspect for that seems pretty far-fetched.
The consumer is somewhat voiceless and powerless here, or if not those, then complacent
("especially since most people in the US don't care about the headlights unless a bulb is burned out"). Remember as well that automakers aren't after the "buy 'em and keep 'em" crowd (Look how many new Corollas Toyota sells every year, even. And those are the cars people KEEP).
There's little internal incentive for automakers to make them better than what is required. They're after the people who get a new car every year or two-- and they get those sales. For those people, what is required is good enough. They'll sell the car or trade it in on the new one-- and the next owner of the car gets saddled with the aging headlamps. And what can they do? They can't either can't afford the new car, or they have other financial priorities-- they'll take the cheap car that runs and has "cold A/C".
Manufacturers will keep getting those first sales, and unless one of the major manufacturers does something differently, setting themselves apart, they'll all keep doing what "works".
Yes, it SHOULD happen that headlamps are built to last longer than they do, but it won't be out of the manufacturers' kindness and it won't be the result of any new laws any time soon because that takes time. And in the meantime, maybe a new plastics technology will be developed that somehow makes longer-lasting lenses cheaper to produce, but don't hold your breath. What's "working" now has been "working" since the '90s when polycarbonate started overtaking glass.