You can visualize it in a larger scale. In a scaled up model, a pieso electric plate is like a trampoline(the working).
Now what will happen if I asked you to walk on a trampoline?
You will certainly need more energy to walk on a trampoline than on a normal ground.
Some extra energy is used to stretch the trampoline, from which I can generate electricity. This same principle is used in pieso electric materials, only in a nano scale.
So where is the extra energy coming from?
The food we eat. The food we eat goes through two processes, first through our own body(where it gets burnt up) and then through the piesoelectric material. Instead you can simply burn the food, and generate more electricity directly, which is more efficient.
So as a summary, you are just changing energy from food produced by farmers, in a very inefficient way, and in a micro level personally(and macro level in total), by investing extra money.
Edit: I would like some expert to advise on this post.