Why is the Remote Desktop not the mobile phone UI, and the desktop is not the mobile phone ui?

Why is the Remote Desktop not the mobile phone UI, and the desktop is not the mobile phone ui?
After the previous blog posts were published, someone asked a question: "If you use a mobile phone as a monitor, why not implement the UI on your mobile phone, but use a remote desktop ."
It is true that the UI tools on mobile phones are more mature and easier to debug.
However, if you do the UI on your mobile phone, you have to write an app. The apps of different manufacturers are different, you must install an app for the smart devices of every manufacturer. After the popularity of smart hardware, the app on your mobile phone is full of smart devices?
Even devices of the same manufacturer have problems with the UI on their mobile phones.
If this manufacturer produces 10 types of smart devices, does the app need to consider the display problem of 10 types of devices for their respective UI? What about the 100 types?
Do apps need to be modified if any of the 10 devices is modified? The number of times you change the version of the app. You have changed the features of a device. What should I do if the features of other devices in the app are accidentally damaged? Is it necessary to change the function of another device? The test workload is greatly increased.
After the change, the user upgraded the app again. The device and app must be upgraded. What if the device is modified and the app is not updated? Or if the app is updated and the device is still an old device, what should I do?
Of course, even if the app can automatically identify the version of the device and intelligently download the corresponding version of the app, the device may not be able to. Some low-end mcu systems do not support on-site upgrades due to costs. If you only have one device, and two or more devices have different app versions, which version of the app do you install? Is the app re-installed before each device operation?
Therefore, only Remote Desktop is the fundamental solution. If you use remote desktop, the mobile phone is just a monitor. If you have installed a VNCclient, everything is done. You can change the mobile phone without having to reinstall a series of apps.