National parks are supposed to be a wonder for the whole nation.  Especially when you consider they come along so late in this game and require four unmodified suitable tiles.  The only national parks I have built were late game when I would build a city next to a natural wonder and put the park there.  In return, you get two amenity in the main city and one in the closest four cities.  

For a national park built on a natural wonder, that seems pretty lackluster so late in the game.

Now, national parks yield 10 amenities to the city that builds it and one amenity to all cities in your nation.  

If I could restrict national parks to requiring a natural wonder, I would probably do that.  But, that is the only spot I ever found for it.

Obviously, you could spam national parks on crappy little cities you add late in the game, but so be it.  Amenities were never a real issue late in game anyway for me.