The variations across Europe (I can't speak for India) are on a completely different level. If you put a random person into two different states in the US and two different countries in Europe for an hour and ask them after which differ more, I would bet serious money they will always pick the two European countries.
Only because of superficial things like language. When it comes to politics and the role of government, Europe is pretty consistent. Barring language and architecture, this makes cities all effectively the same pattern (high density, trains, buses, expensive per sq foot).
The gulf between life in the sprawl of Dallas and the density of NYC is far greater than the difference between Paris and Rome.
My point is that the presence or absence of cultural variation in one place does not equate to a lack thereof in another. Just because someone hasn't seen it, doesn't mean it doesn't exist. Culture isn't based on comparison, it's something in its own right
It's not at all what I've experienced as a US born after living on both coasts, a few different states, and a range of small rural and large urban cities.
Food sure, if you only consider culture as the corporate branded chains. That's missing out on a lot though.
Language, accents, nomenclatures, sayings, mannerisms, all widely varied.
Religions, there are dozens of Christian based sects all with widely varying spectrums of beliefs.
Work culture, still many good-ole boys clubs in certain geographies, pick yourself up by the bootstraps types in others, and anything you can imagine in between.