In short,because of poverty.
I only make US $ 500 a month after graduation. I can barely make ends meet. Yet dating a Chinese girl is expensive. Whenever you are inviting a Chinese girl out to dinner, you are supposed to pay for it in china. Of course, you can suggest to your date that you split the tab. However, from the perspectives of Chinese, asking a girl out to dinner usually means you pay the bill. What's more, Chinese girls usually expects her boyfriend to go shopping with her and to satisfy her every requirement. Plus, Chinese girls see it as boyfriends' duty to buy brand bag, lipsticks and quality makeups as presents. Chinese girls will think that you don't like her if you don't buy them presents on anniversaries, holidays and birthdays. What I hate most about Chinese girls is that they tend to take for granted that you their boyfriends buy presents for them from time to time and they tend to complain about the lack of romance of their boyfriends.
What's worse, Chinese girls not only compare their boyfriends to each other but they also require their friends to possess private car, houses and savings when it comes to marriage, which makes countless Chinese boys refrain from getting married. As a matter of fact, many Chinese boys don't dare to ask girls out and are prone to dismiss themselves as single dogs.