No, that’s not the reason.
If you’re trying to cache just the stuff behind the children which you render with your paint() method then there are lots of ways to do that, e.g. embed another child component at the back that is cached, and which doesn’t contain anything else. Or render into your own Image and draw it in paint().
But if it was all done the other way round, and only the paint() content was cached, that might suit what you’re doing but would make it impossible for other people to cache the result of rendering all the children as well, which is also a valid use-case.