> it's not that difficult to live cheaply for awhile to pursue a business if you are motivated.
The immigrants from poor countries may have the relative advantage that living cheaply feels more acceptable to them, and is more socially acceptable in their community.
Most people spend all the money they make. If the average salary in your community is e.g. $1000 a month, then to look "normal", you have to spend approximately $1000 a month. You can cut corners here and there, but if you do it too much, you will suffer social consequences. Either your friends will start seeing you as a "loser", or you will stop participating at some of their more expensive activities. In such situation, most people won't push too far. Maybe it wouldn't even be a good idea to burn your social capital, because if your business fails, you will need a Plan B.
If you are an immigrant from a poor country, your social circle probably mostly consists of people coming from the same place. They won't judge you for not having expensive hobbies; it's "normal" not to have them. They are probably saving the extra money, and so are you. Maybe you only spend $500 a month on yourself, so if you have a brother, you can make a deal like "one of us goes to work for $1000 and pays for both, while the other tries to start a family business". Or parents work hard and live cheaply in order to provide their children a better future. The second generation is much less likely to live like this, because they already made friends in the majority culture, and start doing the expensive things to fit in.
Social pressure is a thing. It is much easier to live the way people around you do. Sure, it's also about conscientiousness; I am not saying it's either-or, just that "conscientious and living in a subculture that encourages saving" is better than "conscientious and living in a subculture that encourages spending" for starting your own business.