There are plenty of places in the east where land and water are cheap enough that people use it for rice farming. (I have seen rice fields in Shanghai with my own eyes. There's lots of room.) Meanwhile, basic infrastructure in the west is lacking, including the electrical grid. That's also why energy is cheap there: it's difficult to get it to places where people would be willing to pay more.

Guangdong is on the eastern coast. Inner Mongolia isn't, but the main data center hub of Ulanqab was no doubt chosen for being relatively close to Beijing. Guizhou is the one example that's actually somewhat remote, but it's a small share of the total so far. Farther west, Yunnan, Sichuan, Tibet, Qinghai and Xinjiang don't really feature at all in the datacenter business.

Ultimately, the benefits of proximity seem to be more important.