it's really not, looks at guy's in prison, all they do is train, zero supplements and cheap food.
but some of them are huge and get that way in a short amount of time, i know one guy who was inside for a couple of years, he went in tiny, he came out deadlifting 200kg+ and swole.
sure he was getting plenty of sleep, but the main factor was his training, that's all he had to look forward to in his day, so he focussed 100% on it.
now he's out in the real world he only goes to the gym once every 2 weeks and he doesn't put even 10% of the effort he did whilst inside and it shows even though his diet is a lot better and he now takes supplements. therefore he was better off when he was training hard and eating crap than now training at his leisure and taking all the good stuff.
theres even another guy who proved this to be true, he managed to get lean, even though he was eating twinkies every day, because the rest of his diet was spot and the twinkies fitted in with his macros, therefore he proved that diet is the most important thing for losing bodyfat.
there's no point in training hard and not getting enough calories, you need to eat more if your underweight, that is obvious to anyone, but you dont need to put as much focus on the diet, you will still grow if your eating crap, training is the most important thing for growing.
so if you never gained any weight, you werent eating enough, not that your diet was poor, you can eat whatever you want whilst bulking, therefore diet shouldn't be the focus on a bulk, training should.