> t's trivial and cheap to make water harder, home brewers do it all the time to replicate specific styles
I'm aware of this. I did it for my ribbon-winning brews. It is cheap at the home brewer scale, but at commercial scales, maybe not so much.
> can be shipped
Sure, it's possible to get genuine Saaz hops shipped from the Czech Republic to California. Again, A home brewer might be willing to shell out top dollar for a few ounces, but cost and availability is going to be a much larger factor. I've know craft brewers to abandon making certain styles because of uncertain supplies of key hops, and being unwilling or unable to switch to a similar variety.
There's a reason why so many the American West Coast IPA became a style defined by Centennial, Cascade, Mosaic, Amarillo, Chinook, Simcoe, Strata or Citra, and not like English IPAs and their Target, First Gold, English Goldings and Fuggles