Why are they called exam tables?

It’s not really a table. You are not going to sit around it and eat dinner. Tables usually have four legs.

Tables get used for other things than eating on and a table does not need 4 legs to be called a table.

The word “table” is derived from the Old English”tabele” which is derived from the Latin word “tabula” - a board, plank or flat topped surface. Guessing the term applies to flat topped exam furniture.