Larry in TN
En-Route
It's a lot more complicated than that.There's usually the choice of upgrading to business or first, if you don't mind paying more, only usually it's a lot more.
First, many airlines, including AAL, DAL, and UAL, have a premium economy product which offers more legroom. You can buy up to those seats or members of the frequent flyer program who have reached a level of status can get those seats for no additional charge.
Second, many flights offer an economy fare class that come with an immediate upgrade to first class if it is available. It is an economy fare but it is not a deeply discounted economy fare.
Third, many of the people who travel in first class have upgrade from economy fares either complimentary due to their FF program status or by using miles or earned upgrade instruments.
Relatively few passengers pay the published first class fares.
Bottom line is that additional legroom is available for a reasonable cost and upgrades and/or additional legroom is available for those who fly the airline regularly and have earned status and/or miles/upgrade instruments. The quoted first class fares are there for infrequent flyers with the means and willingness to pay for the seats.