There're several reasons:
1. Mainland Chinese airlines are all on a different reservation system (TravelSky).
2. Few mainland Chinese use airline websites. If they need to interact with the Chinese airlines directly at all, they use their apps instead.
3. Unlike elsewhere, OTAs (ctrip, in particular) completely dominate the travel booking business in China.
4. Not unlike the situation with WeChat, there're terrific third-party travel apps (actually better and more integrated than anything available elsewhere) that mainland Chinese use to manage bookings.

So, the bottom line is that mainland Chinese airlines don't put any resources into upgrading their websites/interfaces.