Actually, UCIe does
allow for proprietary protocols. This was certainly necessary to gain widespread adoption by major players. That way, Nvidia can adopt it without foregoing NVLink and AMD needn't abandon Infinity Fabric - it's merely the physical and electrical aspects they'd benefit from. Even then, having standards should help with tooling, IP libraries, and ultimately time-to-market.
There's a good writeup of it, here:
www.anandtech.com
"at the protocol layer, chiplet makers have a few different options. The official standardized protocols for UCIe are PCI-Express and its cache-coherent cousin, Compute Express Link, ...
... the promoters have made it clear that UCIe isn’t locked to just PCIe/CXL. Future versions of the standard may add other protocols if something comes along and the owner is willing to donate it to the standard.
Finally, chipmakers are also free to use their own custom/bespoke protocols as well; they are not restricted to using just PCIe/CXL. UCIe supports a raw/streaming protocol option that allows any other protocol to be used. Both chiplets would need to support this custom protocol to make a connection, of course, but even in this case, this would allow a chipmaker to leverage the physical aspects of the UCIe standard to simplify their own design/production."