Early backpacks just had simple, unpadded shoulder straps made from webbing (woven strap material) or leather, and no waist or hip belt. Initially it was a big deal to add padding to the shoulder straps - first this tended to be wool felt, later foam covered in fabric was alternately used.
When external frames (first wood and then later aluminum) began being used, some clever person figured out that the rigidity of the pack now allowed the wearer to leverage at least some of the weight of the bag off of their shoulders and onto their hips. What started out as a simple webbing or leather belt (not dissimilar to the above mentioned shoulder straps) took on a life of its own when further refinements to the shape and construction of the external frame allowed more and more weight to be supported on the hips: this made carrying what otherwise would have been ridiculously heavy loads relatively manageable. It’s important to recall that these packs evolved to be quite tall and specifically designed (and packed by the user) to maintain a very high center of gravity, which is required if you want to get the weight over your center of gravity instead of pulling you over backwards.
Mid-sized backpacks (called rucksacks back in the day) often had internal metal stays which did allow the transfer of some of their weight to the hips. However, because these packs were intended for ski mountaineering and climbing, they tended to be short and stout, with a lower center of gravity, which made that transfer of weight limited.
Modern “internal frame” packs are a hybrid (read: compromise) between old-school external frame packs and old-school rucksacks: they are taller and trimmer than the old climbing packs, and their internal stays have been amped-up to give a modicum of rigidity to allow more weight to be transferable to the hips. Our Guide’s Pack and our Hero's Journey are examples of this type of pack, though at the less sophisticated end of the spectrum: you can definitely carry a significant amount of the weight on your hips with their padded hip belts.
(An interesting cul-de-sac of backpack design are the completely frameless packs based more-or-less on Don Jenson’s Rivendell pack. He designed a pack with sophisticated dividers that allowed the back of the bag itself to attain a certain degree of stiffness just by loading it correctly. These Rivendell packs were briefly popular in the late 1970s, but most folks found that they were difficult to load just right and that, even when loaded correctly, they did not transfer enough of the weight of the load to the hips. I still think these packs are very cool!)
While small and light daypacks also have waist straps, these are primarily intended for stability (to keep the pack from swinging around too much while hiking or cycling), rather than load bearing, as there is little to no rigidity of the pack to allow a transfer of weight to the hips. That said, some folks are able to utilize even a light waist strap on a small pack to take a bit of weight off their shoulders; if you are able to do this you might find a bit of padding on that waist/hip strap to add some comfort. Hence we offer both a 1-1/2” / 38mm padded waist/hip belt (for the Brain Bag) and a 1” / 25mm padded waist/hip belt (for the Synapse 19, Synapse 25, and Smart Alec packs).
(Shown above: Alpenlite external frame pack with padded hip belt circa 1975.)
Trager Trapper Nelson wood frame pack: webbing shoulder straps, no waist/hip belt, late 1950s.
Alpine Designs rucksack with felt padded shoulder straps and 3/4" webbing waist/hip belt. Late '60s.
Chuck Roast climbing pack: 1-1/2" heavy webbing belt. Early 1980s?
Lowe Alpine climbing pack: 2" heavy webbing belt. Late 1970s.
DOLT daypack: 1" webbing belt.
TOM BIHN large soft-pack with totally over-built padded hip belt! Mid 1980s.
Class-5 daypack: 1" webbing belt. Mid 70s.
TNF external frame pack: padded hip belt with Velcro® closure. Mid 70s.
Mountain Tools frameless climbing pack with integrated padded belt . . . interesting concept. Dig those colors - must have been the 80s.