I've known lots of guys to do that. The reason is simply this: a lot of guys think (mostly correctly) that if he doesn't have his life together and have some stability and some resources, that he isn't going to be able to KEEP a woman around. She might stay for a little while, but she'll eventually need something he can't provide, or just get tired of waiting for him to "get there." It's much easier to do the major sacrificing, the long hours, etc. alone, instead of having a girlfriend who complains that you're never around, that you don't spend time with her, that you're on the road too much, that they're not paying you enough, etc. When you're first starting out, you often have to take crappy jobs or work long hours or be constantly on-call, and the truth is that many (most?) women get insecure about that or feel like you're CHOOSING that instead of being with them.
I'm not saying that YOU are necessarily like that, but so many women are that he has to assume that you are too. If he's gone at work 14 hours a day, you'd never see him during the week, and maybe he comes home and HAS to spend most of the weekend fixing his car or fixing up his crappy home or apartment, and thus goes weeks without spending any quality time with you. You wouldn't be happy. You'd wonder if there was another girl. You'd wonder if he just didn't like you, and was avoiding you, etc. And he would have to deal with all of that stress ON TOP of all the stress of working so much and still having to take care of himself (food, laundry, cleaning, etc.). It can be too much sometimes.
It would be different if you were living together, and you more-or-less knew what he was doing when he was away, and even if he just came home and crashed, you could SEE him sleeping and you'd feel better. But he doesn't have the time to get to that point with you (or anyone else) right now. Maybe things will change, but at least he was being honest with you.