The reason boxing is so effective in a real standup fight (not just a boxing match) is because boxers actually proper spar all the time. Other martial arts, except for wrestling and BJJ/ground game, do the same old practise moves, hit bags, run through various strike & defend scenarios over and over again and think that it's going to help them in a real fight.
I'm quite interested in MMA and enjoy watching the UFC, but I train in boxing and BJJ. In both, we go hard. But I'd say we go harder in boxing than BJJ because we hit hard and get hit hard all the time. Ok we wear 16 oz gloves and head gear, and we don't punch as hard as possible most of the time, but we definitely hit at 90% for power shots. Depending on the sparring partner and what they are comfortable with. Like if I'm sparring with a couple of the guys that can hit really hard, I'll ask them to go at 70% power so that I don't get brain damage and I'm a bit of a pussy.
I've been interested in MMA and quite recently had a chance to look at a few places in my area. A couple of them have consistently produced national level champions too. But when I watch them train it's so different from boxing. Even when they train stand up, they don't go anywhere near as hard. A lot of it is just pretend shit, pulling a lot of their punches. When someone gets hit even reasonably hard it's immediate time out. Really weird shit. For a sport where it's quite brutal in a competitive match, 4 oz gloves, elbows and knees, the training part is a bit weak.
I'm not saying all of them train that way, but I think the vast majority do.
Pro MMA fighters sparring
So called hard sparring by pros
Amateur MMA sparring
looks like complete shit, they're just doing whatever they want, no technique.
Compare this to even amateur KIDS boxers sparring
Sure they aren't big heavy men, but they're staying in the pocket trading punches, throwing punches at speed and sitting down on their punches.
https://www.youtube.com/watch?v=AEyh8WMv1hs
This is why they can take a punch. They're so good at seeing punches coming and they know how to brace their neck for it so they're head doesn't go flying and rattle their brains. The amount of sparring they do means they are seeing several hundred punches coming at them in each training session and their eyes get very good at picking that up and even if they can't slip or block it in time, they can at least brace for it.