Current airliner passenger loads are determined by a mandatory requirement to be able to evacuate the passengers from the hull within a set period.
Cram in extra seats, and you're up against the evacuation limit, which is a hard limit dictated by the number of bodies and the number of doors in the fuselage.
To work around it, you'd have to cut holes in the fuselage for additional emergency exits and then recertify the aircraft, which means an airline wanting to use these seats will need to buy new airliners certified to carry more passengers. But this higher density seating only makes sense for short haul ultra-low-cost tickets, which are less profitable, ergo ...
(And then there's the issue of weight. Passengers weigh, say, 75Kg on average, including clothing. They fly with luggage, another 5-25Kg. A Boeing 737-800 currently has a limit of 189 passengers and a payload of roughly 40,000kg, of which up to 26,000kg is fuel: put more than 189 passengers (18,900Kg by my BOTE estimate above) and you're going to be squeezing right up against Maximum Take-Off Weight unless you reduce the maximum fuel load. Even if there's room in the hull, you're going to have to redesign a lot more than the seats and the emergency exits ...)