This is theoretically possible (SDR) but it's very expensive.

(Before anyone replies about $20 SDRs, those aren't even close to 4G/5G capable.)

Indeeed. Last time I looked these chips were $1300 each in quantities of 1000. Too expensive for even a luxury car.

I think the answer here is an internal USB modem or something. Easy to upgrade. Just up to the car manufacturer to commit to writing new drivers. (I assume that they don't want to do this, so are probably happy when technology changes out from under them. Everyone has to buy a new car!)

What makes this silicon so expensive?

- Car stuff has to be certified in order to be road legal (in Europe, ECE marked)

- Car stuff has to deal with a way wider temperature range than normal consumer stuff (-40 to +125 °C). That is the thing that makes silicon so expensive.

- Car stuff has to deal with vibrations and mechanical stress, meaning more expensive PCBs, less options for solder (remember the infamous NVIDIA solder issue of ~2010?) and connectors

- Car voltage regulation is a hot mess, at least for ICE vehicles - IIRC during engine start, voltages can drop to something like 6-ish volts and after that (or during battery disconnects/other load dumps) into 120-ish volts, and the voltage has an awful lot of ripple because usual generators are AC that's driven through a diode rectifier, with the battery acting as stabilizer.

- Most stuff in cars requires hours of work to replace it - Tesla were to my knowledge the first ones who did software updates outside of shop visits at all.

Regarding the temperature range; this procides a bit more detail and nuance:

https://electronics.stackexchange.com/questions/301826/how-t...

Future-proofing puts you on the wrong side of Moore's Law. Let's say 4G requires 10x the transistors of 3G. In reality that's not a problem because 4G came out 10 years after 3G so transistor costs came down 10x or more. But if you wanted to build a 4G-upgradeable radio years early you'd have to pay for that 10x performance upfront using older and more expensive technology. This same argument applies to future-proofing other tech like video codecs as well.

High complexity, low volumes, I would imagine. A few of the answers below talk about it being a car part but SDRs that can do 3G/4G are targeted at people developing cheaper chips. Electronic test gear is much more expensive than cars (at least at the bleeding edge), so the pricing doesn't preclude it from being used there.