I am just suggesting the rear camera would give much better image quality had they taken a lead from the reverse camera market, and encapsulated the lens in some sort of resin or case allowing for convenient external mount and all the benefits.
reverse cameras are analogue, 480 lines of resolution, and only require very basic cabling (I make IP68 waterproof reverse cameras also), a 1080p camera when extended beyond about 8 inches (as you've done with yours) compared to as done in this product which requires a send and receive chipset, much thicker cabling with more wires inside, shielded cable to combat interference etc, the reverse cameras have a slimline plug inline that is used to put the waterproof part outside and get the join inside, pretty easy to do with an analogue feed that only needs a couple of wires
these aren't exactly challenges that couldn't be overcome but they are difficulties that would need to be addressed, there is one manufacturer that has already made a waterproof external camera but it's quite cumbersome and would be very difficult to mount on many vehicles, for a truck or bus that's probably no big deal, for your average family car you wouldn't want it hanging off the back of your vehicle, the one I'm referring to is already as small as current tech would allow and it's more reminiscent of reverse cameras from 10 years ago in size
now even once you address all that, eventually they'll get smaller etc, for most vehicles where to mount it is the next hurdle, check out reverse cameras and you'll find there are many different designs, this is not because of looks or style etc, it's because what works on one car doesn't always work on others, I do 6 different versions of the reverse cameras now and there's still vehicles that we can't accommodate, translate that into this type of product where you need 6 or more different versions to meet some market demand and all of a sudden this project isn't cost effective
ok so even if you go and make 10 different versions and you can fit it to a wide variety of vehicles, in a great number of those cases it's going to have a very poor view of what goes on, depending on the vehicle design itself as to how dirty the camera is going to get and whether it sheds water or retains it, this kind of stuff if you were building it into the car as original equipment would be much easier to address than when you're talking about an aftermarket one size fits all type solution, the problems are even worse if you tried to do an external front camera
TLDR: putting the camera outside gets rid of the reflection problem but introduces a host of other problems