May be it has a simple reason for not using a cmos-like battery in addition to the actual battery. One might be the consideration that having additional things will cost additional money and is an additional point of failure. You'll need to multiply the costs for a small battery and additionals with your production figures and figures of possibly needed spare parts in the future. All in all, we're talking >1000000 for sure. That's a Lot of money. There also has been a situation with VW 10-20 years earlier. They also decided to change the keys shell shape and shell material going form quality to plastic just to safe a 1-figure cent amounts per key. In the end the decision was an success, because they saved 100k Euro /y. So, a cmos-like battery will cost them..
And also, there is no necessity, like it is with computers. With computers, electrical cords are disconnected each time necessary to without a problem or caution (only it mustn't be running lol). With a car, disconnecting the battery should ideally never be done. So, actually no CMOS battery needed. Once in a few years setting the clock is not that big issue.
The other thing is, it's not only the clock that is resetted :)
Have you noticed, a clock in the car is mostly within the entertainment system? And that's the point why.
If a car battery is disconnected, that happens typically for just a few seconds until a new one is connected. Here, nothing should happen. The clock will be kept typically. If the change takes longer than a certain time, the electrical buffer within the entertainment system will be emptied. THIS is actually on purpose.
It's a countermeasure, it should make stealing & selling of the entertainment systems not so easy anymore.
Because, with longer power loss it loses it's entanglement with car's hardware, too, and needs a code to be entered after new power is connected. That's a good thing.
The clock is A. within that system and will be powerless, too, and B. It's a visual signal, that there has been a power loss recently. One should check.. may be the wires, iduno.
So and that's the reasons for not having a cmos-like battery in your car.
A. Money saving,
B. Visual info on "electrical status" and
C. countermeasure against thiefs.