It would be incredibly hard to actually make the game diverse. Even if they made it American and European, that wouldn't be diverse. What about all the other continents? And on top of that, buildings and roads look very different even from country to country.

They would need to make too many different roads and building types. So, for the base game, they have to focus on one, maybe two themes, make them either seperate or mix them together. If you want something else, you can either hope for DLCs or wait for the new mods and assets to come to the workshop. The game has always profited greatly from its huge modding community.

And just in case you are a console player or you don't like to use mods ... the game is built to be modded, to make your own assets, and so on. It's developed with that in mind.