Why...
The simple answer is, because most of the people running Zombie Survival games prefer freeform. Why is a different question...maybe the people playing/running these games want a bunch of Daryl characters. Maybe they are using 'freeform' in the sense that there's not a specified rules set...but not in the sense that anyone can write anything into the game and it becomes part of the storyline (a lot of freeform games are actually 'limited' or 'moderated' freeform...) After all, 'freeform' and 'sandbox' are not necessarily synonymous terms.
But, really, what it comes down to is, they get the gaming experience they desire from a freeform game. Even in most of the system-based games I've been a part of, the GM tends to keep dice-rolling to a minimum, in order to keep the game rolling along smoothly. And in a few games I'm in, while they're described as freeform, the GM actually does have a mechanic in place for conflict resolution, but only resorts to it occasionally (if you describe what you're doing in such a way that he sees no logical reason it would fail, he rules that it succeeds...)
Both approaches have advantages and disadvantages to them...and it comes down to individual perception as to whether the advantages of one outweigh its disadvantages strongly enough to sway the choice clearly to a written rules system vs. freeform vs. something in between.