I'm no expert, but from what I gather, it's the available chipsets that determine the feature set.
There are a limited number of chipsets available, but they are not all the same. Yes, the boards based off those chipsets are often repackaged and sold under a number of different brands, but bigger companies often roll their own with different designs and of course different firmwares. This is very important for Mac compatility. My guess is that a lot of the generic ones do minimal Mac testing because I often find them problematic on Macs in one way or another.
As for ones with multiple USB-C ports, apparently the chipsets available that supported that were few and far between, partially because there wasn't much call for them on the Windows side, at least until recently. Plus there are bandwidth and power delivery concerns that need to be addressed.
This is why I much prefer having the ports on board the Mac itself than having to rely on a third party hub. However, cost is of course an issue. If the choice for me is between a $1299 Mac mini and a $1999 Mac Studio, I'd buy the Mac mini with hub because I don't want to spend the extra $$$ on the Mac Studio, I don't need the power of the Mac Studio, and I don't like the size of the Mac Studio.