While there isn't an official RFC yet since the protocol is still in the RFP stages[0], the amount of new cars on the road will require roughly about the spectrum allocation as what's needed for cellphones right now. If we tried to cram all of this into existing 5GHz range of lower frequency spectrum, it's simply not enough. 5G has both a larger spectrum allocation and throughput of data available to it.

Edit: The cost of putting in a lidar system for your car won't be too expensive (probably <$1000 when done across a manufacturer's whole fleet); but think about having to pay for another cellular line on your phone bill at current telecom prices. It would be prohibitively expensive to the end users for mass deployment.

[0] https://www.nhtsa.gov/sites/nhtsa.dot.gov/files/readiness-of...