Wider seats take up more space, which means fewer seats which means less revenue. And there's no way they'll give that up; they're facing competition both domestically and internationally now from sardine-can aircraft layouts on low-cost carriers.
So these days, US-based airlines have standardized on a width of 17-18 inches in economy class. Used to be long-haul international flights would have slightly wider seats, but now they're aiming to get 10-across seating in economy and you can't do that with 18-inch-wide seats.
If you want a wider seat, your options are to pay for first class or (if traveling internationally) pay for premium economy on a foreign carrier.
So these days, US-based airlines have standardized on a width of 17-18 inches in economy class. Used to be long-haul international flights would have slightly wider seats, but now they're aiming to get 10-across seating in economy and you can't do that with 18-inch-wide seats.
If you want a wider seat, your options are to pay for first class or (if traveling internationally) pay for premium economy on a foreign carrier.