Even my toothbrush uses an MSP430, while the simplicity of it's task could easily be done by a 4-bitter.
In this modern world there is no relation between what it costs to make something, and the amount of money you have to spend to obtain an object.
According to octopart, you can get the small MSP430FR2000IRLLR for 28ct in 1000+ quantities, but I'm guessing you can get them quite a lot cheaper if you skip the retailers and buy millions of them directly from the factory.
The padauk uC's is probably as low as you can get price wise. I think they went as low as 3ct, and I think even those are 8-bitters.
Another guess is that the market for 4-bitters is too small to reach really high quantities, and therefore the more universally usable 8-bitters may be cheaper.
Setting up a semiconductor factory is an expensive business, and to make a profit with 10ct parts, selling millions of them is not enough.
If a 4-bitter is only available in some old process technology and it's sleep current is too high, this may be prohibitive. Saving 5ct on a uC does not help if you need to spend an extra 10ct on a bigger battery.
Recently I saw a documentary from ASML, and they claimed that around 95% of the equipment they ever made (from the '80-ies) is still in use, and I believe that.
Once you make equipment very accurate, keep it clean (cleanroom!) and have regular maintenance, it does not wear anymore and life is virtually infinite. With a perfect oil film there is no mechanical contact, and thus no wear. It's small imperfections, oil contaminated with dust and such that cause wear. (Edit: apparently no oil in semiconductor equipment, but the general principle still stands: better fits in higher quality equipment lowers wear and increases endurance)
In the end this means a lot of the lower-tier stuff is made on quite old equipment, but I don't know how that relates to 4 versus 8 -bitters.