Not being extremely mobile or having chronic pain does not require you to eat more calories than you burn. I have a torn disc and take nerve meds so I obviously don't lift weights or run like I used to, but Im allowed to moderate my calorie intake.
Age and a slowing metabolism are more pernicious, but even those things don't "force you to eat more calories than you need". Nothing is forcing anybody to do that.
I believe I read people donate much less if money is involved. Part of the motivation is the altruism of the donation.