> react passes everything as attributes for some reason

Because that was their design choice: if you write attributes you pass them as attributes to the components

> I personally don’t understand why this hasn’t been fixed.

Because React is very popular and it's used everywhere in Facebook. You can't just change the behaviour without breaing a lot of stuff

> Preact has managed to support properties

> it would be fairly easy for React as well.

Yup, they should "just" do it. Meanwhile, here's an overview of what's needed for that support: https://github.com/facebook/react/issues/11347

Preact uses Option 2 in the list. Other libraries go for either Option 1 or Option 3. React is going with option 2 in an experimental branch.