Because overly focusing on weight issues can impair the relationship between person and dog. If a dog is basically healthy, a little overweight or underweight, but happy, able to do everything s/he wants, the owner enjoys the dog's company, they can do everything they want together, that's a much better situation than one where the owner thinks about the dog primarily in terms of "not good enough" or can't look at hir dog without thinking about food, without thinking that s/he's a failure as an owner, not walking the dog enough, feeding it too much, etc. It can get in the way of the relationship and enjoying each other's company all on the alter of someone else's idea of the ideal dog weight.
If the dog is NOT happy, is in pain, cannot do the things s/he would like to do, doesn't have energy, isn't healthy, that's another matter. To not address diet and weight in that situation would be neglect - when my dog stopped eating to the point he got dangerously thin, I did something about it, tried a hundred new foods until I found something he'd eat and gain weight and have energy on again. My cat is still having problems with mobility, so we're working on diet, restricting access to other food sources (actually, the dog's change in diet has helped quite a bit with that) and encouraging play (which is a fun thing anyway), and very slowly he's regaining mobility. But will he ever be "ideal"? No. And that's ok. I can't force him to exercise, and I can't feed him little enough that he loses weight dramaticly without causing him to be lethargic with lack of energy (which would be counterproductive anyway).
If an ideal weight can be attained and maintained with little effort and little negative impact on the family's happiness and the relationship between human and dog, then it would be neglectful not to do it. But if food and weight becomes a struggle between human and dog, makes it so neither of them is happy and the relationship isn't enjoyable, then choosing to accept the natural variations in pets' weight can be a rational decision that caring, intelligent pet owners can make.