I feel like I'm missing something really fundamental regarding the cross section of the photoelectric effect. I'm looking at this chart, and it seems that the lower the energy of the incident photon, the more likely a photoelectric interaction is to happen. Higher energy interactions get less likely, until a cut-off point, at which they no longer occur.
If the energy of the ejected electron is equal to the incident photon energy minus the binding energy, I understand why a lower limit exists (since there must be enough energy to eject the e-), but why should there be an upper limit?
Why does the cross section reduce with energy at all?