Why do # of camping supplies need to increase at higher levels? High level characters still eat the same amount of food as low-level characters.

If it's to balance the fact that high level characters have vastly more wealth and you're trying to keep rations at a similar fraction of total party wealth in order to limit long resting...well that's just an indicator that limiting rests solely by food (that you can freely purchase from merchants) doesn't work. Especially when you can just "send food to camp" and thus don't have to worry about its weight.