US regs do allow replaceable lenses, but only if the headlamp meets stricter requirements in terms of reflector durability. The idea being, a lens probably won't be replaced the minute after it breaks/fails, so water and dirt will get into the headlamp and the reflector will have to be cleaned before the new lens is installed. Therefore, replaceable-lens headlamps have to pass an inspection (no visible damage allowed) 48 hours after their reflectors are wiped with bug/tar remover (45% xylene, 55% petroleum base mineral spirits), or plain mineral spirits, or other non-water fluid as specified by the maker for the task of cleaning the reflector. And they also have to pass a test where the headlamp, without lens, is subjected to a 24-hour salt fog/spray, allowed to dry naturally for 48 hours, then the reflector gets cleaned in accordance with the maker's instructions included with the new lens, the new lens gets installed, and the reassembled lamp then must meet the photometric requirements.
So, makers have every incentive not to provide replaceable lenses: it requires that the headlamps be made with more damage-resistant reflectors and it requires additional testing, both of which add cost. It's much easier to get a good and durable permanent lens/housing seal than a removable and resealable one, which not only means extra cost but also extra warranty repairs (more extra cost). And repairable headlamps means fewer sales of replacement headlamps, which means less profit. All of that extra cost and reduced profit in exchange for nothing but the happiness of what is probably the vehicle's second, third, or fourth owner.
Headlamp lens degradation kills people. That's not an exaggeration, it's been proven, and NHTSA is aware of the proof, but hasn't done anything about it. Practically, lenses that are replaceable and/or adequately durable are the right thing to do. Economically, from the car/headlamp company perspective, not as much, at least not in the world we live in, where quarterly results and short-term shareholder value are about the only thing that matters.
With all that said, things are slowly getting better. SAE is working on figuring out what and how to specify in terms of better coatings, and LED headlamps are a lot easier on their lenses than halogen and HID, because with LEDs the lenses run much cooler (heat accelerates the degradation). Of course, LEDs also bring along their own new material-degradation problems, such as yellowing/opacification of optical materials used in close proximity to LEDs, but those problems are being effectively addressed, too.
It's fun stuff, all of it...and it argues pretty forcefully that the standardized sealed beam concept (leaving aside any particular implementation and just looking at the concept) was very close to optimal for vehicle headlamps.