Vehicles don’t only transport people, they also transport data.
Autonomous vehicles are much more connected than they used to be. A lot of data is exchanged between the vehicle and the clouds, you don’t want all that to go over 4G/5G. Instead, data transfers are scheduled to appropriate times. This way the data transfer can be transformed from a cost center to a value center.
They use networking protocols that are independent on the underlying technology, so they can use anything e.g. DSRC, Wifi Mesh.
OpenWRT is essential to create such a vehicle network.