Why is it not easy to stay and work after studying in the United States?

It depends on your major and your own ability.

The employment rate of the majors studied shows a gradient distribution. If you are majoring in business or humanities, it is naturally difficult to find a job in the United States. But you studied engineering, especially the first gradient computer science. Recently, artificial intelligence and deep learning are booming. You can easily find a job anywhere. Even as far as I know, many business jobs require you to learn deeply.

In addition, compared with Native Americans, outsiders are a disadvantage. Under the same conditions, why not look for native Americans and China people? However, as long as the value you can bring to the enterprise is greater than the cost that Native Americans need to pay on you, why not hire you as a businessman and a manager of the enterprise? So your ability is also very important.