In addition to the other answers, there is a point where the space available to a passenger is adequate, and adding more wouldn't justify the additional cost to said passenger. I'm sure airlines have done extensive research on how much space people are willing to pay a premium for.

Fewer seats, fewer tickets, fewer dollars.

Cost.

I just want to add on top of other replies, for people with that money that do want that kind of space, they would either hire or own a private jet.