[farhanito]

Because they did not implement it, as they see no value and no reason in it?

Chromecast is the best example, that even THE company who created all these APIs and created Android does not fully implement the available features. Bad for the end-user, that's it.

The API exists and is officially documented, if a vendor (be it google or whomever) decides they don't want to implement it, then it's not there. Easy as that. Having this API is not mandatory for an android box in general, but it's a mandatory requirement if you want refreshrate switching to properly work on Android with standard apps.

Most likely they will add it with some update if people complain loud enough. Why not writing them an E-Mail. Tell them about your two broken boxes and that you had hoped that someone would do it right ...

I know Android 11 comes with new API that let apps to request the preferred  refresh rate and the system decides if it is possible to do so based on many factors. I wonder if Kodi will adapt to that new API instead of current way of enumerating all possible resolutions and refresh rates, white list it, then actively switch to the matching refresh rate?

On my Shield and FireTV Stick, I don't need to white list anything. Matching refresh rate just works.