People underrate the homogenous culture that makes it easier to build something and have a large market that will consume it.

In Europe, for example, you have a much less homogenous market. A French retailer/business may be popular in France, but has a lot of friction going to another market. Not so much in the US.

So large, homogenous, wealthy culture that, if you can address a problem for, creates a financial foundation, and makes your app exist in at least a cultural context somewhat portable to other markets outside the US.

“get the consumerist culture, higher willingness to pay, etc”

Who cares if you’re the king of some country in Africa. Manhattan has more money to slush around on apps than the entire continent combined and that’s literally just one area of one city in one state in the USA.

The primary reason is the rule of law.

You keep mentioning Brazil in your comments, but as a founder I'd rather not have to keep track of the various government officials I have to bribe in order to be compliant, and even then, not be completely sure.

I hate lawyers and I hate governmental red tape, but at least in the US it's consistent, and the rules are laid out for you ahead of time. Finding out three months after launch that you have to buy Chico's cousin a hooker in order to get your business license renewed is suboptimal.

"but there are so many other markets where you can build and sell much simpler products and dominate. "

You need to clarify this with real examples. Which Markets ? The reason everyone wants to be in US Market is because of the size, adoption, maturity of consumers (willingness to spend) and many other reasons. Yes there may be markets where it may be easier to replicate but they have limits and ultimately you want a piece of the US Market.

The infrastructure for setting up and running a business in the US is significantly easier than most places. Not all places, but the combination of a huge English-speaking market and the ease of doing business makes it more attractive than many of the other places mentioned.

There are certain businesses (social media) for which people believe scale is everything. If you believe that then the U.S., India and China are the markets that matter. (e.g. you don’t see a social media site from the Netherlands so Israel make it big globally)

$$$.