Because they cost more and people won't buy them. I commuted, year-round, by bike for 19 years. As I recall, my NiteRider Pro light cost around $300-$400. (The current NiteRider Pro Endurance is $449.99) It had high and low beams. There were also instructions on how to mount it properly to show the trail/road. Part of my commute was on roads/trails with no lights and I wanted good lights.

I knew plenty of people that thought a $15 light was fine, as well as a number who said they didn't need lights.