The purpose of a table is typically to sit at or to place something on... Now, you can sit at a pinball machine but the only characteristic that is similar between the two is that they both have four legs. I mean a "chair" typically has four legs but you would never call a chair a table would you?
From the historical perspective, it makes more sense to call a pinball machine from the early 1900's a table because they were almost entirely made of wood and the tops were flat, even if they had glass covering the playing surface.
So, the historical pre-woodrail pinball era term has carried over to modern day, even though most of the "table" characteristics have been lost.