Seems likely they simply don't sell enough full-cost unsubsidised phones direct to consumers to warrant a separate firmware for that stream. Not surprising, phones are expensive these days, and almost everybody is on a contract for their SIM anyway, so why not get it from the carrier?

I don't, but I know I'm the weirdo. The only other people I know who buy phones outright are my brother because he keeps dropping them and smashing them, and a mate who buys my old iPhones when I upgrade every 2 years or so because he knows I look after them.