US and British packs typically did not use a hip belt, shoulder straps only. In the US hip belts didn't really appear on general use non-skier packs until the 60s. You're up in New England, not far from the users of this (LINK) and the tumpline...

The hip belt really came in after World War II, European Alpine and Nordic Ski Troops used rucksack / bergens (from the Norwegian town where the ruck with hip belt was refined) to achieve that stability Dayuhan mentioned. For a large load in rough terrain, it is very beneficial. Those benefits led to increasing adoption from the 60s until today. Much of that stemmed fom the WW II US Army Mountain Rucksack, with frame and belt -- a first AFAIK for the US -- developed for the 10th Mountain Division and used also by the 1st Special Service Force among others and which was adopted postwar by Special Forces due to its huge capacity. The influence of Europe and increased international travel also played a part. The rest, as they say, is history.

I think the number of small packs which won't carry as much as 30 pounds or so but which have belts is a a fad more than a necessity...

IIRC, the rule of thumb is that about 35 pounds makes the use of the hip belt worthwhile. I'm fairly good sized so my cut off was about 50 pounds for a belt to be desirable, less than that it wasn't necessary and was in fact an annoyance. I never used one with my ALICE.. I also was able to carry that much or more only very, very rarely. IMO, most people carry way too much junk...

There are a lot of day packs that don't have belts. Check Mountain Hardwear (LINK) for one, Kelty for another -- Kelty has one called the Shrike (LINK) which has a removable belt and I think I've seen some with no belts.