I would guess that there is a desire to not create too many product tiers. I believe 6 core parts are made from 2 3-core CCXs, (rather than 4 and 2) so only one core is disabled per ccx.
Current Ryzen and EPYC processors have 8 core CCXs. The 6 core parts used to be as you described, but are now a single CCX. The Zen C dies have two CCXs, but they are still 8 core CCXs, and are always symmetrical in core count.
The big exception is that the new Zen 5 Strix Point chip has a 4 core CCX for the non-C cores. I think the Zen 4 based Z1 has a similar setup but don't remember and couldn't quickly find the actual information to confirm.
It would be sort of cool if they could do direct to consumer sales with every core going at whatever its maximum speed is or turned off if to disrupted. But that's not something you could do through existing distribution channels, everyone presumes a fairly limited number of SKUs.
