I think your view of China is kind of skewed. Hong Kong is far from the only developed city in China. Cities in these eastern areas, where the population is densest, are developed, and people there lead normal lives by our standards. In fact, in a lot of those cities I would say life is better, but that's my personal bias. I love Chinese food and beautiful lit-up areas like most of Shanghai.

People want to leave China for various reasons. In the western parts I'm sure it's just as you say - they want better lives. People in the eastern parts probably leave for the same reason that we enter. They want to travel. English also happens to be a universal language that brings tons of career opportunities to them if they know it, and living in an English-speaking country is a great way to learn. Also China's population is very homogenous, and I'm sure people want to have some variety.

And crapping on the streets? Okay, I saw that once in Shanghai in my 170-ish days there. It was really weird, and not anywhere near the walkway. Shanghai also has world class education, the best transportation system ever, an actual maglev train in commercial use, tons of arcades, and this, so there's that.