Yes. It’s especially interesting to hear people say this when some states in the US are larger than some countries in Western Europe. For example Texas is slightly larger than France, and Montana is slightly larger than Germany. Of course, crossing state boundaries in the US does not introduce you to as much diversity as crossing many country boundaries in Western Europe. However, when you consider there are 50 states in the US, it should be reasonable to understand there can be substantial differences in habits and lifestyle.
I hear this argument a lot, Americans insinuating their different states are effectively almost different countries.
It is nonsense. Americans are Americans. Of course you have regional differences, no different to people from York and Bristol would. But you're not as diverse as you think.
Outwardly I'm sure Brits, Australians, Chinese, Indians etc all get lumped together in your head, despite each of those countries having extremely differing internal cultures spread across the respective countries. That's how the rest of us see Americans - LA and New Yorkers are not different, except in subtle ways only obvious to you.