Odd question but looking at the market, I see Tamron/Sigma making lens for first-party companies such as Canon, Nikon and Sony.
For manual lens such as Zeiss ( for Canon/Nikon) I see that they aren't competing with Canon/Nikon directly since all the lens provides is EXIF/IPTC data so that wouldn't impact the native lens that they sell directly. (Also, the price range is different (Otus) or they are made for video (Milvus)).
Is there a reason why Canon and Nikon allow third party makers to make lenses that provide similar specs to first-party lens such are AF, IS and aperture control when they sell for less? It seems that Canon/Nikon could block Sigma and other 3d party lens makers just with firmware updates, not that everyone would be happy about it.
It seems that Tamron/Tokina does license Nikon lenses.