I think that's because of Chinese people's travelling habits. Popular domestic travel destinations include Hainan for a tropical experience, Sichuan for pandas, Beijing for landmarks, Hong Kong for fake Britain, Macau for gambling, and Taiwan because it seems foreign enough without being actually too foreign (to Chinese people).

Hong Kong, Macau, and Taiwan require travel permits to enter, despite the Chinese government considering them "domestic". They kinda straddle the line between actual domestic and international. Regardless, it's not common for Chinese people to have phone plans that work in Hong Kong, Macau, or Taiwan, so they'll buy the disposable SIM cards I talked about earlier. That's why phones typically have two SIM slots.

Getting visas to travel internationally is a pain for Chinese people because they have to visit a consulate or embassy, apply, and then be subject to high scrutiny. After all, it seems everyone's scared of Chinese spies nowadays. It's also very expensive by Chinese standards compared to applying for a cheap HK/MO/TW travel permit. The People's Republic of China passport is pretty weak compared to European or American passports. Chinese people can get visa-on-arrival or visa-free access in South Asian countries, Central Asia, or Africa, but these destinations are not popular with Chinese tourists.