It's entirely reasonable for WASM to use its own IDL. Afaik Chrome already uses an IDL to describe the interaction between native C++ classes and their Javascript bindings.

Integrating WASM modules with each other, WASM with JS, and WASM with native functionality seamlessly is a huge and unsolved challenge.

Or it just points towards NIH syndrome, combined with hype and anti-hype driven mentality of programmers, and general forgetting of previous technologies. Or even when the differences are close to 0, doing a new not exactly same implementation for ... whatever reason.