I think they don't include them because they simply do need to.
They could quite literally have no improvements made to Leo's (I was going to write their but really the they that work at Fender didn't design the models that made them popular) designs with the exact same current appointments and the same level of quality control for the next decade and they will easily continue to sell.
If they want to increase sales, it can be as easy for them as offering some different paint colours particularly those which have been used in the past but are uncommon.
Like the green apple colour was cool, and the dark night colour option was even cooler.
OP I get your point though, they could add a small and cheap improvement, but it's not likely. Change happens really slow for Fender, they took an absolute age to release a 5 String Precision.
For a company that use to be a leading innovator they sure do seem to be resistant to innovation, I think it's quite sad especially when the founder was a continual tinkerer, striving to improve his own designs, maybe that's the reason, the true source of innovation for that company sold it and moved on.
The six string bass in it's B0-C3 configuration has been around for quite a while, most manufacturers offer six string models in their range, maybe fender will have a release date for a 6 string Precision at NAMM 2060. I could be wrong but I think Fender have made only made two B-C 6 string models, the first being for Tye Zamora and the second being for Steve Bailey, I'm not sure if Tye's model was ever for public sale but I think the Steve Bailey one was available for a short period of time.
One day there be abundant string retainers for all, Fender B-C 6 string basses with 24 frets, and some successful new models designed by the current staff at Fender. That's a gonna be a good day.