Because the Chinese audience (and market) is galactic. Your drama doesn’t even have to do well across the board - if you have 0.5% of the population watching it, you’ve already got a huge bonafide hit. Popularity in China translates to a plethora of lucrative deals everywhere else.
A kpop group made one casual mention of a shampoo brand on a Chinese variety show and sales jumped over 600%. Imagine how much foreign and domestic companies would pay you for a CF deal if you can have that effect on the Chinese market. Lee Min Ho was paid 900k USD just for a 15 min appearance (via video! not even in person!) in a Chinese program.
You don't have to be paid a lot for shooting a Chinese drama (although I can’t imagine their pay being low). The drama doesn’t even have to be good. But the exposure it gives you does increase your earning potential exponentially.