They haven't even bothered to try. 3" more legroom for $40? No. Human-sized seat? I'd pay double the price. But I'm not going to pay 5x as much.
You'd think there would be a logical in-between, but in a world where 20% more space in business class costs you 5x as much, there's no room for the average customer to have a pleasant flight.
Sure they have. All three of the big legacy US airlines have some form of extra-legroom economy: American has Main Cabin Extra, Delta has Comfort+ and United has Economy Plus.
The problem is those seats cost more, and people shop for airfares based solely on the price they see in a search engine.
Airlines have tried all sorts of tricks to get people to buy the nicer seat. They've tried showing it to you as an option at booking. They've tried offering it as an upsell afterward. They've tried offering it during check-in. They've tried branding it as a separate fare class labeled "premium" but still a reasonable-looking price.
It's entirely due to passenger behavior at this point: people will grumble and complain and whine about the legroom and the lack of power and the extra fees for everything, but the next time they book a ticket they'll still just sort by lowest fare and pick that.
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.
Most airlines also build free or discounted access to those seats into their frequent-flyer perks, though Delta has recently begun cutting back hard on that and treating it as a possible-but-not-guaranteed after-booking "upgrade" even for their top-tier customers.