If PRC (china) doesnt recognize ROC (Taiwan), why do I transfer via international channels in Chinese airports?

Basically title. When I was traveling to Taiwan through China, I was told to go to international airport (Shanghai) and through international channels (Guangzhou). Why? Why is it not domestic? Why, if the entire world doesn’t recognize Taiwn, and China applies One China rule and constantly claiming its their territory and province, why do the recognize its sovereignty in airports?