It's the sum of everything in the backpack that counts. Laptop, charger, bunch of dongles, water bottle, whatever you might want to carry around. That's why the focus on biceps in this thread is a bit odd, people don't usually carry their laptops around with their hands all day.
I am not at all saying you need to go develop enormous biceps or even any muscle at all, just baseline movement and core strength a few days a week for most people without certain conditions or the like of most age groups* shouldn't have any problem with a 15lb backpack (that's 12lbs of accessories including the bag, which is WAY more than nearly anyone needs for all day use and then some with a 15" MBP).
But, yes, you're absolutely right, so the problem is *not* actually the 3lbs MBA (b/c how much more are you going to pull out of that without hiking the price, killing the durability, or the battery life?): it's all the other stuff people carry around. Outside of the extreme edge cases, please, tell me what one would need that a basic, baseline level of fitness most individuals* can very easily (almost without any real work) can achieve that is going to be more than 4-5 additional lbs? Even carrying a 26000mAh powerbank (I would argue one does not even need this, though, because inside that 3lbs is a generous battery that will run it for most of a day unless you're blasting After Effects with simultaneous ProRes renders, etc, etc - in which case, uh, well, maybe don't use an MBA and check your expectations on what 3lbs can give you b/c, well, physics....), a notebook, the weight of a bag itself, charging brick, lightning cable, and a lunch you aren't getting much over 5lbs unless your notebook is made of lead, your bag is soaking wet with mercury, you're using lead-acid batteries, your lunch consists of a 48oz porterhouse and 2lb baked potato, and carry all your currency only in pennies. Then, well, yeah, you got me there!
So your point about carry and accessories is well taken: which is why I said consult an athletic trainer, because even the most enormous biceps aren't going to be useful with a 15 lb backpack all day (not just the wrong muscle, but strength vs basic functional fitness/endurance), because for an "able"-bodied person*, carrying even 15lbs in a backpack should not be a problem with even the smallest attention to their health, wellness, and fitness even just three days a week.
I think those pointing out the fitness aspect of this* are actually the only sane ones here and have a good point that at some point, spending nearly all our time hunched over screens on our rear ends in the most sedentary of lifestyles is actually the problem, not a 3lb MBA in a 15lb backpack (and if you're not sedentary and have no problem carrying a 15lb backpack in your day to day, then, well, you're not part of this subpopulation, so don't worry!).
*Again, this intentionally excludes those who have suffered accidents, health issues, or born outside the "normal" idea of "able" and that is something that we should intentionally ensure we make accessible all the ways we can, and I think Apple does pay attention here