That would be fine if there was enough space for everyone to bring a full size carry-on. There isn't. So charging for checked bags causes too much carry-on, and you end up checking bags for free. So why not just check them for free anyway.

It might be because gate checked bags are limited-release, but I find that immoral unless the bag is being checked because of the wrongdoing of the passenger, not the wrongdoing of the airline.

Neil