It can be a lot of things. Some people just have no motivation, but I think most of the time it's either a disorder, the way they grew up, or a health issue. For example, I grew up as a computer nerd because I had no energy, ever, and later discovered I have food allergies and low blood sugar and have to carefully manage my diet to feel good. After I started feeling good and having energy to do things, life became a lot more fun!
However...I have a friend like the one you are talking about. Had a lot of the same problems, although he did show up to work (retail) - but everything was like pulling teeth, bare-minimum effort, non-stop complaining, etc. Then I met his mom. Holy cow. I'm surprised he turned out as functional as he did. I think environment can have a huge impact on people, especially at a young age. Verbal abuse can be killer. Bad examples can be killer. His situation is probably the first time I realized you can break a person's spirit without ever laying a finger on them.
Chronically sleeping in late and constantly playing video games, in this particular situation, is escapist behavior. He doesn't want to deal with the reality of life and is doing his best to ignore it. He sounds depressed to me - depression can remove your desire to do pretty much anything, but since you still have to exist and since suicide is painful, avoidance behavior is the next best option. Staying up all night puts you in a haze where you don't have to deal with your problems because you can't think straight, and video games help keep you distracted.
So for starters, I'd say depression. And immediately coupled with that is poor care of your body: staying up late, not eating healthy, not exercising. So he isn't feeling good emotionally, and now he's not feeling good physically, which creates an endless loop...it's hard to feel like doing anything when you don't feel good on a regular basis, which in his case is probably a lifetime. So he's probably never really taken the next step to figure out what he wants to do in life, and to take part in a lot of the fun things in life.
It's really difficult to help people like this because they aren't in a position where they really want to be helped, because they don't feel like it and thus don't want to deal with it. If you can help him bootstrap himself by altering his diet and getting him to go out on a walk with you every day, his motivation might start improving a bit, but it's also hard to overcome a lifetime of habits, and if he's not used to working or doing much of anything, he's not going to want to change that right away.
My friend eventually got his own apartment, but only because his family kicked him out. His roommate has to cover most stuff and he still has kind of a difficult time with life. It's hard to see people like this because their life could be so much better if they just decide to change and then do it (easier said than done), but their mindset & health put huge barriers in front of them for doing that. Unfortunately I don't have much advice to give you, other than to say I understand...my friend could be a really good guy, but he just can't pull it together, even to take opportunities that would help him. It's a tough situation. Wish you the best.