Here we go again.
Technology prices change over time. What was too expensive/impractical at one moment might be viable at a later moment.
Your smartwatch isn't a 400MHz Pentium II requiring a 150W power supply running on AC current.
At this very moment, Apple has not marketed an 8-core Mac mini, but there is a 6-core model available for sale.
Ten years ago, there wasn't even a 4-core model. As time progresses, the price of technology tends to drop.
Right now, the currently available 8-core CPUs from Intel haven't appealed to Apple's gross margin cost analysis. But Intel will release more CPUs over time (search for "Intel roadmap" online some time) and the cost of processing power will continue to drop.
In the same way, someday the Mac Pro will probably have a 48-core option. But not next week.
Could Datsun have installed an airbag in their cars in the late Sixties? Well, maybe the technology was there but not quite affordable.
Trust me, technology doesn't stay static.
Anyhow there's a strong likelihood that there's a Mac mini with an 8-core CPU running somewhere in a secure lab in Cupertino. There are probably Macs running macOS on ARM processors. There are probably ten different devices that are vying to be the Apple Watch 6.
Just because it's not for sale today doesn't mean that it can't be a possibility in the future.
Remember, today >80% of Macs sold are notebook models and all of them have built-in WiFi. This wasn't the case 25 years ago. There wasn't any WiFi back then anyhow.
Macs themselves are only a fraction of Apple's total revenue; thus the importance the desktop Mac in shrinking. You can see it in the Apple Stores and how much real estate is given to the various products. I doubt if the Mac mini generates even 10% of Mac revenue. Apple clearly doesn't bother to update the Mac mini every year, unlike the notebook product lines.
Adding new CPUs will come to the Mac mini, but slowly and infrequently.