The comics say it's "just outside Gotham City" and in "Bristol Township", which is why it has extensive grounds and nobody's explored the subterranean area and found the caves that Batman uses. If it were in Gotham proper, SOMEBODY would have tried to put utilities or a subway tunnel or something either through it or close enough that the caves would be common knowledge. And when it's in movies, it usually has a LOT of space around it.