Because it doesn't. It is roughly the same functionality spread out into separate and typically more expensive boxes with more cabling and power supplies.
What these "make it as small as a Mac mini" designs do is strip out functionality and that incrementally add it back in by stacking boxes on top that had what the Mac Pro had in the first place.
The number of CPU packages doesn't really increase at all. These improverished but inexpensive interconnects most often purposed here don't allow for mulitple box NUMA CPU set ups. So no there are no more CPU cores any more than Intel's tick/tock updates allow for.
There are boxes that are designed to maximize x86 core count but they are primarily server boxes. It is extremely unlikely there will ever by a Mac with those characteristics. They likely will be a Mac Pro though that doesn't maximize possible x86 core count without any regard for cost, but does do a decent job of balancing GHz and core count to turn in substantively better than previous generation preformance. No multibox modularity needed.