American Airlines used to offer more legroom in economy several years ago. They found that given the choice, people will buy a ticket at a lower price on a competing airline rather than pay a little bit more for more legroom.
You do have a choice, you can pay more for economy+, business or first.
I'm 168cm and weigh 66kg. I don't need a bigger, wider seat and I would not be happy paying more for my airfare just to satisfy larger people than I.
Edited to add. There are rules relating to the the maximum number of passenger seats on an aircraft. I don't think anyone is proposing that these rules be liberalised.