The Qt doc says,
As mentioned, each program has one thread when it is started. This
thread is called the “main thread” (also known as the “GUI thread” in
Qt applications). The Qt GUI must run in this thread.
The Android doc says,
Like activities and the other components, services run in the main
thread of the application process
And iOS,
It is strongly recommended not to update UI controls etc from a
background thread (e.g. a timer, comms etc). This can be the cause of
crashes which are sometimes very hard to identify. Instead use these
to force code to be executed on the UI thread (which is always the
“main” thread).
Why does they use a single threaded model to update UI ?
The short answer is, it’s the only reasonable way to ensure that the display is not corrupted.
The long answer is that allowing multiple threads to update the UI results in deadlocks, race conditions, and all sorts of trouble. This was the painful lesson taught by Java’s AWT (among other UI systems) that allows multiple threads to touch the UI. See, for instance, Multithreaded toolkits: A failed dream?. That post refers (via dead links) to Why Threads Are A Bad Idea and Threadaches.