The popular OSes/kernels are too bloated and don't have wide embedded support (and porting to new devices takes considerable time), and the few free embedded OSes don't have much traction (and aren't used on more powerful platforms). Would be nice to have a middle ground.

For example, FreeRTOS doesn't support 64-bit intel arch. And you don't "ship an app on FreeRTOS", it's more of an API and framework you use, and you sort of write a module in C and compile one big app. Quite different from non-embedded app design/shipping. You won't be able to run an Android app on an ESP32, but it should be possible to write apps for ESP32 and run them on Android-compatible hardware. But FreeRTOS would need optional MMU support, and you'd need extra components to load the app, in addition to hardware support.

If you're asking "why would you do that", it's because I want to write simple purpose-built apps without all the trappings of a larger OS and run them on all types of hardware. You could technically build a 'smart watch' that isn't so smart but runs on a single battery charge for 1 year. But not if you use a power-hungry SoC. Want a more efficient SoC? Good luck figuring that out. Making that whole process easier unlocks more technical solutions and products.

There are countless different RTOS. The most polishing one is Zephyr. FreeRTOS is more like "Scheduler" than a proper OS.

Also am i correctly assume you want an OS that's truly "cross-platform", that can run on every architecture ? Or you just want to "have an app on ESP32 to run on Android" ? Because the latter can be done with proper abstraction layer.