New cars sold in the US from about 1940 to 1983 were required to have sealed beam headlights - at first, two 7-inch round ones being the only option; later, four (separate high and low beam) smaller round ones, and later still, two large or four smaller rectangular ones.
Those were necessarily glass, contained an internal reflector, and the front glass was heavy enough to be impact resistant. They were mostly easy if tedious to change (front plate plus retaining bracket, each with screws). They had the advantage of each one including all the parts, and being like a lightbulb in that all the internal parts are totally weatherproof - and since they were all otherwise fairly traditional incandescent bulbs (even if halogen), they burned out, and the whole sealed beam was replaced.
By 1983, under pressure from auto makers pointing to cars in the European market's reasonably successful use of non-sealed-beam headlights with few of the problems seen in pre-1940 headlights (including corroded reflectors, etc), those were allowed in the US too.
For probably all three of the reasons in the original question (cost, impact resistance, weight) with varying priority, the front clear part was usually plastic. That may have gotten better over the years, but I'd guess that unless they've gotten a lot better, they show their age after a decade or so, depending on climate and usage.
Buses, emergency vehicles, commercial heavy vehicles, pickup trucks (to some degree), etc, still favor sealed beam headlights (even if they're not incandescent and not one continuous piece of glass except where the wires come out), probably for reliability and ease of maintenance, and because style is less of a concern on such vehicles.