Because it is anything but simple.

Years back, senior year design project was exactly this. There are a millions ways to harvest energy, but to do it economically is incredibly difficult. To do it efficiently and reliably makes it that much more difficult. And then storing the energy to be used later adds even more cost and complexity.

The money spent on trying to generate power from tidal waves is ultimately better spent on other methods. There are (or at lest there were) test wave generators in Spain or France (if my memory is correct) but I don't think they were ever truly commercialized.