I suppose the only answer possible is "Because the mechanism that was evolved requires it."
Total energy is less important here than whether a single photon has sufficient energy to cause the ionization/charge-separation which is the first step in photosynthesis.
And that single photon doesn't have enough energy in itself either; it takes four photons to cause a reaction, while the energy is 'stored' in the manganese ion cluster of Photosystem II.
It's all a very intricate system, and it's somewhat telling that nature has only evolved this once; it's essentially the same PS2 in all green things.
Perhaps, for instance, a lower-energy photon would not cause a large enough charge separation, and the electron would too easily return back to the chlorophyll.