WebP did not always support lossless compression. It’s conceivable that the tools’ developers made the decision before that.

WebP was first released in 2010, and lossless support was added in 2011. I really doubt there were tools built between 2010 and 2011 that haven’t been updated since then.

That does not fit the use case of diagramming tools. They usually have comparatively few assets that are used multiple times in the same document.

SVG is probably a better fit for this use case.