Professor Oak is in fact the strongest Pokemon in the world. Because of this, he already knows everything about Pokemon (and doesn't have to care about trivial things like what Gary's name is). He disguises himself as an old man, and for kicks and giggles he only shares parts of his knowledge at a time. Together with his lackeys, the other Professors, he entices countless young children to leave their homes to train endlessly with Pokemon and "catch 'em all" until they become unstoppable. These children, wielding all the world's legendary Pokemon, are at his beck and call. Because of this, Professor Oak pretty much owns the world, so of course he doesn't have to tell the people of Kanto in Gen I about the rest of the Pokemon.
