Did the OP somehow ask for free association and I missed it?

I suspect that even in the high strength steels, .4-ish is as thin as they feel comfortable drawing the tube. The heavier gauges are for when people want to build a stronger frame, for example a touring frame. Or for big people. For that matter, some of the CE testing requirements for frames are hard on a steel frame, so maybe that's the reason these higher strength tubes are available. Production bikes are often built heavier out of (possibly undue) caution, and it seems that there are 853 production bikes out there.