It's because the replies are interpreting the op's question differently from the intent.
When op asks: "why there isn't a standard protocol?" -- he's asking "why isn't there a SingleDominantThatWorksOnOnEveryDevice audio protocol that lets me connect devices seamlessly?"
The op's word of "standard" is just doing a lot of heavy lifting to convey a frustration with stuff not working intuitively.
The analogy is TCP/IP being a standard (SingleDominantThatWorksOnOnEveryDevice) network protocol that won over Apple AppleTalk, Novell SPX/IPX, and Microsoft LANMAN NETBIOS.
But many replies interpreted "standard" as "any available existing specification regardless of marketshare or device availability" -- so that's where you get various examples of audio protocols that are idiosyncratic to particular domains which are not analogous to the ubiquity and reliability of TCP/IP. E.g. the Dante audio protocol which doesn't seem relevant to op's use case.
And what's the scope of an "audio protocol"? Is it a "media query of music files" protocol like DLNA? Or is it a "virtual hardware audio device endpoint" like Bluetooth Audio?