Why didn't the Nujiang River turn 72 build a tunnel?

The terrain is steep. The 72nd turn of Nujiang River is a famous Devil's Road in China and a Panshan Highway, with 72 continuous bends and steep mountain roads, which is a headache for drivers on the Sichuan-Tibet line. The total length of the whole road is12km, from the lowest point to the elevation of 3100m, climbing all the way to the highest point, Yelaguan at an altitude of 465 1 m, and then spiraling down to Bonda Town at an altitude of 46544. Because of the steep terrain, it is really not suitable for building tunnels.