First of all, when it is said that 'dark energy is independent of time', this means that the density of dark energy is invariant, not the total amount, which does indeed increase with the size of the universe. Of course, "dark energy" or "quintessence" in the simplest models are just cosmological constants. I.e., the same "constant" that appears in Einstein's equations. Another way to get a constant contribution to the energy density is to have a scalar field which is resting at the minimum of its potential. Many popular inflationary models are based on this picture. From the perspective of elementary physics, however, the easiest explanation is that dark energy is constant because of it's equation of state. The equation of state says that the energy density is equal to the pressure, which, one may easily show, implies that the energy density is constant as the universe expands. (dE=pdV, therefore dE/dV = E/V, implying E is proportional to V, or that the energy density is constant.)
Incidentally, one might ask why the positive energy density doesn't cause the universe to collapse. Instead, dark energy fuels the expansion. The answer is that pressure outweighs the energy density in Einstein's equations since there are 3 directions of space and only one of time and pressure is related to the spatial directions while energy is related to time. This means that the pressure is basically three times as effective at expanding the universe as the energy is at contracting it, resulting in a net (accelerating) expansion.