Prolly because at one time books were written on tablets, and the word table comes from tablet. Trying to find what was written inside the book, or what the book contained, prolly meant looking at the little tablet or table and what it contained or contents. Hence the words, "Table of Contents." I am surmising this, and haven't researched this, so I could be wrong.
It's just a common term, used for so long it's become a standard description throughout the world in writings & reports. It is not unusual for organized lists of all kinds to be referred to as "tables."