Explain why there are more substances on earth than are listed on the periodic table of elements.

There are more substances on Earth than are listed on the periodic table of elements.because substances are combinations of the elements. Think of it as if the elements were the alphabet (A, B, C, D...) and substances were words.
we can make a very larger number of combinations with the letter of the alphabet, likewise, you can make a larger number of substances by combining elements