This is a great question. The setting of Of Mice and Men is ironic, because Steinbeck describes the setting as beautiful and tranquil, but as the story progresses there is nothing beautiful or tranquil about life for the migrant workers in general and Lennie and George in particular.
The opening lines of the book set the stage, the scenery cannot be more perfect. There are trees, a body of water, and beaten paths where children have played. The picture that emerges is a land flowing with milk and honey. Listen to Steinbeck's description:
There is a path through the willows and among the sycamores, a path beaten hard by boys coming down from the ranches to swim in the deep pool, and beaten hard by tramps who come wearily down from the highway in the evening to jungle-up near water. In front of the low horizontal limb of a giant sycamore there is an ash pile made by many fires; the limb is worn smooth by men who have sat on it.
Based on this description, you would expect a very different kind of novel, not the one that we read in Of Mice and Men. The contrast between a setting of plenitude and the dearth of resources for the migrant workers is ironic, to say the least.
However, if we are astute and pay attention to details, there are clues that the setting might be misleading us. A comparison of the first and last chapters is illustrative. For example, in the first chapter, there is a water snake gliding effortlessly in the water. In the final chapter that snake is swallowed up by a bird. In other words, there is death, but it is only hinted at. For the most part, the setting is perfect, perhaps too perfect. Something bad must happen, and it does.