The argument somehow came to my mind yesterday, and I'm not sure it's true either. But do you really think human value might be as easy to maximize as pleasure or pain? Pain is only about internal states, and human value seems to be partly about external states, so it should be way more expensive.
One of the more crucial points, I think, is that positive utility is – for most humans – complex and its creation is conjunctive. Disutility, in contrast, is disjunctive. Consequently, the probability of creating the former is smaller than the latter – all else being equal (of course, all else is not equal).
In other words, the scenarios leading towards the creation of (large amounts of) positive human value are conjunctive: to create a highly positive future, we have to eliminate (or at least substantially reduce) physical pain and boredom and injustice ...