Curiously, TSMC seemingly published their N7 defect densities and they're low enough that most chiplets would not have outright dead cores. Specifically, they said 0.09 defects per square cm in a slide you can see at https://fuse.wikichip.org/news/2879/tsmc-5-nanometer-update/ . If that's saying what it appears to, lower SKUs must use a lot of chiplets where all the cores turn on, but (say) might not hit the top chip's performance spec within its TDP.
The 7742 needs all cores to run at 2.5GHz averaging ~3.2W apiece if you leave 25W of the 225W for the I/O die. The 7642 is looser: 2.3GHz averaging ~4.1W apiece, and that after dropping the "worst" core from any CCX where they all work. (For non-obsessives, a CCX is a four-core group connected to a 16MB chunk of L3 cache.)
Note lower SKUs like the 3700X/3800X and 7232P use 8-core chiplets. You can figure the chiplet count for an SKU by dividing its L3 capacity by 32MB, and from there you can figure how many cores each chiplet has enabled.
There's also plain market segmentation, i.e. enabling/disabling stuff on identical chips to sell at different prices. In this gen I doubt it's good strategy for AMD to hold back much performance like that, though, since they really want to get some market share right now.
(If turned-off cores generally work but below spec, that suggests there could be some way to make them useful for extremely-threaded workloads. Split hardware threads onto two lower-clocked physical cores when it looks like a net win, say. Can see enough potential thorns not to bother trying, but thinking of possibly-useful silicon sitting there turned off makes it just so tempting, heh.)
If so, maybe AMD doesn’t have high-enough yield on their 64-core part (i.e. 8-core chiplet sub-part) to satisfy huge bulk orders for them, without also generating huge numbers of the 48-core-binned SKU (i.e. 6-core chiplets, really 6-out-of-8-enabled-core chiplets) in the process.
And I would suspect that their production process is such that they do have a real, explicit 6-core chiplet part as well, which can be mixed-and-matched within a single CPU with the flawed, re-binned 6-of-8-core chiplets, giving them a powerful hedge on their own logistics (in about the same way that SPAM has flexibility in their ratio of chicken to ham that lets them ride out turbulence in either market, making the end-product cheaper than either input), but requiring even further that people consume the SKUs containing “6”-core chiplets.
I would bet that AMD very much wants to sell large buyers the lower-core-count CPUs, since their yield guarantees that—at least for now—they have so very much more of them, and attempting to make more of the highest-end part ensures that they end up with even more of the less-than-highest-end chiplets laying around.
AMD probably ideally wants order-flow of CPUs in a ratio, e.g. “1x 7742 : 8x 7642”, and offers both better deals monetarily, and far faster delivery (/less contention on orders with other clients) when you take them up on it; or when you buy huge numbers of 7642s alone, such that you’re consuming the cast-off from bullheaded clients who wanted pure 7742s.