I think it's probably the small bursts of reward in the form of the "delivery completed!" screens. Personally, I tried it twice. The first time I gave up at the end of ep2, the second time ep7-ish I think. I really liked the game engine and the controls were great. But unlike MGSV (which I really enjoyed except for the fact it was essentially unfinished), I found myself mildly irritated by the constant deliveries, and found the enemies more just an annoyance than a threat. The story was also just a bit weird/thin for me (and I liked the MGS story, more or less).