Based on the limited info we have, I would guess that he's a perfectionist. I've known a couple of people who have ruined great ideas and initial traction due to their perfectionism.
I think it's even worse than perfectionism. It's a fear of success. Some engineers have this fear that they've only got a few good ideas in them and once they finish with this one, they'll never have a second one, so they've got to milk this one for all it's worth because once they're done they'll be all used up.
This guy is trying to milk it, because he thinks if he gives it up he'll have no more good ideas for the rest of his life. Mark Z. on the other hand is doing just fine and could start trying to do space travel like Bezos, but he doesn't really lose anything doing what he's doing because it's working.
But there isn't any milk. Zuck is milking it. Zuck could do other things, instead he continues to do Facebook and similar acquisitions. He is milking it.
This other dude isn't getting any milk. He's pulling a teet with nothing in it.
Perhaps it's that he believes he is eventually going to succeed and he's convinced himself of this since he also consciously or subconsciously believes that if he doesn't he won't have any other ideas so he might as well try forever to get this idea to work.
You can see an image of his later iterations later in the article [1]. It does look like there isn't much improvement between them, except what a perfectionist might find to tinker with.
