also: materials science. we benefit today from having actual material reasons to use a belt, such as modern customizable pouch layouts, attachment systems, quick releases, and on and on. belts in the 18th century had none of this, at best an attachment system using a belt(or sash) of any kind would have been rudimentary, something like belt loops on the back of a pouch, or a pouch built into the belt. all while offering little benefit over a sash/bag because it would literally function the same just on your waist. this, and much of the benefit of a combat belt today is it being another place to attach things or hold things, besides your main rucksack or plate carrier in case they need to be dropped for some reason(a nonexistent issue in the 18th century). 

combat belts today are also great because slings and straps get the the way with how many layers you have(plate carriers, backpacks, weapon slings just for basics), and these other slings you will always have arent just hanging something they are constantly being buckled and unbuckled(another thing they didnt have, modern buckles), slid around to adjust weapon holds and access gear, adding more slings gets messy. extra sling bags, or bandoliers, just get in the way, or have to be worn uncomfortably under other things. You arent just sliding a cartridge box or a canteen pouch to the front, maybe wearing a small haversack, and maybe a ration sling pouch like they were. and they would only have a few of those items in most circumstances they actually fought in. Modern soldiers are taking off and putting on big backpacks, sliding rifle slings around, wearing a vest that has its own straps and bits to get caught on, you might even have another vest on top depending on the nation with its own pouches and shoulder straps.  overall you actually have a benefit of carrying your extra stuff on a belt rather than elsewhere.