Because mechanical keys are not a good enough security measure.
See: https://en.wikipedia.org/wiki/Kia_Challenge for what happens if you skip the immobilizer
See: https://en.wikipedia.org/wiki/Kia_Challenge for what happens if you skip the immobilizer
It would be reasonable to have something like key and pin (similar to chip and pin[1]), where you have to enter a PIN and have the physical key to start the car. Or have a fingerprint reader. Or use the alertness camera for face authentication.
Since nearly everyone has a phone, and that phone is likely paired to the car, it should also be sufficient to sense that the bluetooth of the phone has paired to the car and at a sufficiently strong signal strength (and is not a new pairing).
The idea that the key itself must embed cryptographic hardware is a historical assumption that should be reconsidered.
[1] https://en.wikipedia.org/wiki/EMV#Chip_and_PIN_versus_chip_a...
Also your steps are about starting the car, but entering the car itself usually needs security too. And since people like unlocking cars remotely that leaves us with either the current RFID keyfob, or some app over Bluetooth or mobile data. The app is great but it requires a phone and is easier to put behind a subscription.
Lol no it wouldn't. That's not making it to production any more than BMW's heated seats subscription charge.