I have never thought of location and a building as a perk. It is easy for even a moderately funded company to have an office in the downtown of any major city.
A perk is not a perk because of the ease of providing it or its cost. It's a perk because it's a benefit not part of core compensation. And it is true, none of the other tech giants have downtown HQs (although plenty of Google satellite offices are in urban centers, at least).
There is a formal definition, but not everyone will agree building and location are perks.