What if they are happy at a local optimum, and what if we human could only feel happiness on a scale between the best we've been and the worst we've been?
Sure then they may not be the happy at a maximum, but since they haven't found such a maximum yet their hapiness on a relative scale is 100%.
In other words, ignorance is bliss, and if you don't have "other experiences", it doesn't matter, you could still be just as happy - or even more!!
Considering I felt like the author, happier in the most work intensive years, and less happy afterwards, I have come to this theory of my own. It's not substantiated by anything but the famous "ignorance is bliss".
It has had one effect - stopped me from seeking "experiences" - because I want to maximize my relative happiness level. I noticed that when I did stopped that (buddihsm like), happiness happened (but didn't last - maybe there are disminishing marginal returns of staying at one level of "experience" - and we should gain experience, but little by little, to keep happiness maxed out.