No, you didn't miss anything. It's just that... for years, we got "browser detection is bad" thrown at us. I realize I'm conflating things a bit, but personally I never did anything like "if useragent=='mozilla' redirect to 'we only support ie.html'", although many did (back in the day). "browser detection" got a bad rap, and had been "out of favor" for a long time (or appeared to be). Yet... the only way to avoid sending down a 400k of HTML to a mobile device is to... detect the device and send down smaller amounts of stuff.