(1) you don't have enough time to helicopter all of them, some will just have to deal with their own problems
(2) having one disappointing kid isn't the end of the world. you have others to shift your expectations and hopes onto
But if you only have one or two kids... you simply can't afford any failures. If your one kid fails, you're a failure of a parent. So you spend as much time and (maybe misguided, maybe not) energy on them. I think this is a natural response, which makes me think it's pretty un-fixable unless family sizes increase.