It was originally a key feature in the arma series that was still enabled in the DayZ mod. And since there are obvious advantages using it (as you mentioned) people continued using it either because they liked it or (like me) they didn't want the 3person-guys to get the upper hand. And since people got used to it, they're not very willing to part with it (I think Rocket suggested once that he was thinking of getting rid of it entirely but I think he realized that people were to fond of it). Anyway, if you (like me) wanna play 1person, just join the right server. There might not be as many, but I can promise you that there are enough to have fun :)