Delaware will become the 22nd state to legalize recreational marijuana

> But Australia and Canada and the US are all English speaking—do they not warrant being separate countries?

Canada is also French-speaking. There are some big differences in attitudes between the US and Australia on various topics - for example, Australians have a mostly settled cultural consensus in favour of gun control and liberal abortion laws, to give two examples, unlike the US where both are very heated political conflicts

> What about Australia and New Zealand?

New Zealand actually participated in the negotiations for forming Australia (as a federation of British colonies), but at the end decided to stay out-if history had turned out a bit differently, it might have been an Australian state not a separate country. In a lot of ways the differences between Australian and New Zealand culture are relatively trivial - different accents, different words for certain things. You get the same differences between Australian states, albeit to a more limited degree.

However, there is one big difference: over 17% of New Zealand’s population have Indigenous (Māori) ancestry, and Māori have a common language and a great deal of common culture-leading to a significant Māori influence on mainstream NZ culture. Australia’s Indigenous peoples are less than 4% of the population, and divided among numerous languages and nations, which means their impact on mainstream Australian culture has been significantly smaller.

> Britain and Scotland?

One way in which the US is very homogeneous is that state politics aren’t particularly distinctive from national politics. Every state is the same two parties facing off against each other, the main political difference between states is the relative power of the two. Whereas, each of the UK’s four constituent countries have different party systems - the SNP only exists in Scotland, Plaid Cymru only exists in Wales, the major parties in Northern Ireland are different from those in the rest of the UK. Spain, Canada, India and Italy are other examples of countries in which region-specific parties are big players in politics. The only real example in the US are territories such as Puerto Rico, and if Puerto Rico ever became a state, it would likely start converging with the American political monoculture.

Politics isn’t the same thing as culture, but political culture is part of culture, and greater political diversity is generally both a symptom of and simultaneously a promoter of greater cultural diversity.

> That's my point--just because places speak the same language doesn't mean they're culturally the same

Yes that's true. But I compare different Australian states – yes there are cultural differences even there, but they are pretty minor. If I compare US states – I'd say somewhat bigger cultural differences than between Australian states, for a bunch of reasons.

But still, my impression is, the cultural differences between US states aren't really that big by global standards. If you ranked countries by degree of internal cultural diversity, the US wouldn't be at the very bottom of the list, but it would be a long way from the top – I'd even say closer to the bottom than the top.

In some other countries, you have state/province/regional nationalist movements. Hard nationalists want their state/province/region to be an independent country – such as the Scottish National Party or Parti Québécois. You also have soft nationalists, who don't ask for independence, but seek greater autonomy while staying in the federation – such as Quebec's current ruling party, Coalition Avenir Québec; or the former ruling party of India's Punjab state, Shiromani Akali Dal. The US has a few state-based nationalist movements, but they've never been remotely mainstream, none of them have ever controlled a statehouse or a Governor's mansion. The existence of (practically significant) regional nationalism is a sign of greater interregional cultural difference, so countries which have it (such as Belgium, Canada, India, Spain, the UK) generally have significantly greater internal cultural diversity than those which lack it (such as Australia or the US).

> American politics isn't homogenous at all--that's just an artifact of political narratives being filtered through mass media in New York City. To use your abortion example (because I agree politics is an expression of culture):

Okay, but on the other hand that actually is an example of American politics being homogenous – because the whole country is arguing about the same things, of which abortion is a big one. Compare that to the UK – the Protocol is arguably the biggest political issue in Northern Ireland politics at the moment, in England it is a lot further down the list of people's concerns. A never-ending political debate in Quebec is about how to balance the promotion of the French language with the rights of the Anglophone minority – a topic that isn't very interesting to the average person in British Columbia.

Your example is just that in some parts of the US, the national abortion debate doesn't neatly line up with the two national parties. Meanwhile other countries have completely different debates in different parts of the country, and even completely different party systems in different parts of the country. All are examples of diversity, but your example is a little bit of diversity compared to other countries which have a lot lot more.

Also, your point about "mass media in New York City" is another example of homogeneity – US media is very centralised, national, tends to focus on national narratives more than regionally specific ones – that media homogeneity is both a symptom and a cause of cultural homogeneity. The BBC has its own division for Scotland – many people in Scotland (especially nationalists) complain that BBC Scotland is too London-focused and doesn't provide enough coverage of Scotland's own politics and culture – but even if those complaints are valid, no doubt it is doing a much better job of being regionally-focused than the US media does.