Everyone will have their own versions of the rest endpoints, their own version of input params, and lots and lots of docs scatterd.
A standard, will help the ecosystem grow. Tooling, libraries etc.
Either the AI can figure it out, and it doesn't matter if there is a standardized protocol. Or the AI can't figure it out, and then it's probably a bad AI in the first place (not very I).
The difference between those two possibilities is a chasm far too wide to be bridged by the simple addition of a new protocol.
Having A2A is much more efficient and less error prone. Why would I want to spend tons of token on an AI „figuring it out“, if I can have the same effect for less using A2A? we can even train the LLMs with A2A in mind, further increasing stability and decreasing cost.
A human can also figure everything out, but if I come across a well engineered REST API with standard oauth2 , I am productive within 5 minutes.