Well, I agree that the farm is somewhat safe right now.
But the problem in all Zombie-related things (movies/Tv-shows) is that one mistake is enough, one can screw up which attracts a herd of zombies to the farm, and then it's not so safe anymore.
Sure they had put up some barbed wire (?) or something, but it was sorta in the middle with forest all around, so if zombies were to come from all directions, things might get tricky.
As for why they hadn't looted any of the cars on the highway, I agree that it's a bit strange.
The most possible reason as I see it, is that they're a bit afraid of going too far from the farm, plus the fact that the freeway would most likely be one of the more common places to run into zombies, then let's say.. chillin' back at the farm.