You must realize that while data storage is getting larger and cheaper, you are talking about the automotive market where components have to operate and survive through wide temperature ranges and vibration. The devices on the consumer market are much more advanced in this respect.
Also, in the automotive market you're a hero if you can slash $0.50 from a BOM, so putting unnecessarily large memory in your design would need a very good business case. For instance, the original language in the California AV legislation stated that the vehicle would have to record 30 seconds of all sensor data leading up to an incident. In an Autonomous Vehicle this is a very loaded statement, as these cars have LOTS of sensors generating massive amounts of data every second. If you were to record full - resolution video for a camera system you could possibly pass 1GB every 10 seconds. Some of these cars have 8+ cameras, 6+ radar's, probably LIDIR, on them. You can see from this that you may have to decide what data to save with some discretion.
Don't get me wrong, if people start buying cars based off a dash cam being an available option then you'll probably see it happen. But right now the climate is more about privacy and big brother than this.