Those are the only relevant locations for somebody who is considering emigrating for career purposes. The other countries do not have job markets big enough and salaries high enough to merit the effort of moving there, considering we are talking about actual countries, with different languages and cultures.
Because most of the companies are American and like to keep their best guarded talent close to home, I guess.
Also the corporate culture is entirely different. In a European IT company you won't get paid $600k for being an IC. You might get that level of pay, but as some kind of management. But as just one of the runts who does the IT? No way.
