I think the craze started with the 375 H&H that had very little shoulder to use for headspace and actually required the belt to properly headspace. When the 300 H&H was released the belt was left on for marketing purposes and people associated the belt with more power. A whole host of "magnum" cartridges were then introduced with the belt to appease the public's desire.
The craze continued up to the introduction of the Ultra Mag and Short Mags (Lazzeroni beat them both to the punch but never really grabbed the marketshare).
As you noted, most people reload to headspace off the shoulder and never seem to complain about the unnecessary belt on the bottom of their cartridge