I think it's the opposite: glass is really cheap, which makes up most of the window. Less expensive than rebar and concrete and the labour for the form work and such. Certainly the frame can be pricey, but most of the surface area is glass.

So glass is a popular facade because it saves the builder money: CapEx. But of course glass sucks an insulation the other side of the equation is heating/cooling: OpEx, which the builder may not care about.)