Why does a doric order in Greek arhitecture doesn't have any base?

Answer 1

There are a couple of possible answers. One is that the lack of a base is simply a defining characteristic of the Doric order. It would be something else if it had a base.

Another reason would be aesthetics. The Doric is a robust or "masculine" order, according to the ancients. As such, a base was seen as unnecessary; it was something more suited to the "feminine" Ionic order, or the "matronly" Corinthian. Those orders certainly benefit by being separated from the horizontal elements of the rest of the building; the entasis of the Doric column looks something like a flexed limb, and thus does not need to be isolated from the other elements.

There is also the whole question of ratios and proportion; a base on a Doric column would seriously mess up the basic ratio, and the Greeks and Romans took those things seriously.

Answer 2

Because the ancient Greeks and especially the Dorians, believed that beauty was in simplicity.