I understand you have been riding for many years, and know what you talk about based on your own experience..
But i really dont understand your hate against backpacks. Have you been using the wrong type or size of backpack?
I'm not really sure why you'd have to understand. Can't you just accept we're not all the same?
But OK. if it helps...
I find ALL backpacks:
Uncomfortable - (granted some are worse than others.)I have used some of the best (incl EVOC) while working as a coach/guide and still can't say I ever enjoyed the experience.
Heavy. - and all the extra weight is high up. this makes a huge difference. to your bike's performance (suspension, braking and handling) and also to how dynamic a rider can be when riding.
Hot/sweaty - surely this is self explanitory. But being too hot means you sweat more and require more water/food on a ride. I can generally ride without a pack solidly for 3 hours with no water/food. with one I'd be drinking tons and need to re-fuel too.
Restrictive - not only when riding but also when bailing from a crash. Ninja's don't wear 15l rucksacks do they? for good reason
Tiring - see the last three points.
They ALL "look" shit. Yes. I'm a vain poseur. I'm an ex DH racer of course I care about how I look on a bike.
But above all else. I don't actually need a back pack at all on any ride ever except when working guiding/coaching (where it's mandatory)
When riding in my own time I carry minimal spares, tools and minimal water/food
2x superlight tubes, 4x Co2 and head, multitool, puncture/patch kit/zipties in a little Dakine hotlaps gripper frame bag fitted on the bike. (All of this wouldn't fill a 500ml pack nevermind a huge backpack). I'm also a professional bike mechanic/guide/first aider so pretty confident in the condition of my bikes and pretty confident I don't need most of the shit loads of folk think they need to carry on every mtb ride.
Waterbottle and emergency snack I carry on a waist holder (the type runners use). Phone/keys/cash in pocket.
I rarely even wear a helmet, NEVER wear gloves. Only ever wear any sort of body armour/knee pads if riding DH uplifted. and it'sjust knee pads even then. Don't ever wear a jacket - I'm well known for this. (just dress for the conditions.. usually underdressed compared to 99% of other riders). I just don't get cold. Part reptile possibly
hope this helps you understand where I'm coming from somewhat