Because most of the world uses JSON and has rich tooling for JSONSchemas, notable many LLM providers allow JSONSchemas to be part of the request when trying to get structured output
LLM providers allow sending any string of text though, right? In my experience the LLM understands XML really well, though obviously that doesn't negate them from understanding JSONSchema.
No, it's more than just text now, it's more than just an LLM for the most part now too. They are agentic systems with multiple LLMs, tools, and guardrails
When you provide a JSONSchemea, the result from the LLM is validated in the code between before passing on to the next step. Yes the LLM is reading it too, but non LLM parts of the system use the schema as well
This is arguably much more important for tools and subagents, but also these things are being trained with JSONSchema for tool calling and structured output
Why would we need to concert XML, which already supports schemas and is well understood by LLMs, back to JSON schema?