RTC’s are not inherently accurate. You have an RTC in your computer, but disconnect it from the internet for a year and it’s extremely unlikely it will be able to pass an OTP check.

Add to that the fact that RTCs run off power, means that the fob would need to actively pull from the battery 24/7. What happens when that battery voltage drops below the required power level? The time goes out of sync. Not to mention you need to change the battery at some point.

Adding Bluetooth would be a terrible idea. You’d then need to make sure the device can receive firmware updates, and we all know the reputation car companies have for updates to things.

Better option would be to receive the date and time from a transmission, be it FM or cell. And no, you wouldn’t have to pay for cell.

The problem comes when you’re in an area that doesn’t have these available, which is still quite possible in the US.

So we loop back to these not really being that viable. They will work most of the time, in most instances. But they’d be shooting themselves in the foot in certain places and with certain users.