That's not necessarily the case. New York City does host some world premiere events at theaters like the Ziegfeld. I lived in New York City for 5 years -this was back in the late 1980's and early 1990's. While some of the theaters there are pretty good (or were), I've seen other locations out-do the projection and sound quality I experienced at NYC theaters.
General Cinemas Northpark 1-2 theater in Dallas had the best presentation quality of any full time commercial movie theater I had ever visited. The #1 auditorium had over 1000 seats. Its THX certified sound system was one of the very first assembled (personally configured by Tomlinson Holman himself); it was one of a handful of theaters that had THX for the release of Return of the Jedi back in 1983. No theaters in New York City had THX at that time, just theaters in Dallas and Los Angeles. The Northpark 1-2 ran lots of 70mm Dolby mag prints (Century JJ projectors). It was one of the first in Texas to have Dolby Digital. The theater had a huge run with Jurassic Park in DTS. It even had a couple 70mm DTS shows (Vertigo and Titanic). The crew at the Northpark 1-2 regularly inspected and re-tuned the sound system, often each time the theater played a different movie. Screens 1 and 2 had projectionist balconies that also served as VIP seating. Lots of filmmakers sat on those couches, including Steven Spielberg, George Lucas and Ridley Scott. The Northpark #1 screen could probably go toe to toe with just about any acronym labeled, premium priced theater in this country and win easily.
Sadly, the Northpark 1-2 was closed in 1998. The theater building was demolished not after that, to make room for expansion of Northpark Mall. I could only image how good Dolby Atmos would sound if the Northpark 1-2 was still around and Atmos was installed inside of it. There's something lacking EQ wise in stadium seated theaters. I don't know what it is about them, but nearly all have a thin and hollow character to them. Their sound doesn't have that chest-pounding, meaty yet clean "oomph" I experienced at the Northpark.
AFAIK, Dolby's personnel have been performing or at least supervising all of the Dolby Atmos installations that have been completed so far. If a certain movie theater already had a great sound system, then Atmos might not seem to add so much more to the experience -especially if the movies are mixed in a conservative manner where they don't sound noticeably different than plain 5.1. A theater that didn't have a good sound system before would require a pretty substantial upgrade to run Atmos -particularly in terms of demands on surround speakers.