I don’t really see how it works both ways. It’s hard to imagine a bigger scarier competitor to Samsung than the Apple iPhone, yet we agree Samsung is happy selling screens to Apple.
I recall reading a breakdown at one point a few years ago that indicated that between screen, fabs (before Apple went for TSMC) and memory, Samsung made more money from each iphone sold than they did for each galaxy S phone of the time. Seems like a winning deal to me.
Because there are a lot more Androids and other devices that need screens. If they didn't make screens for Apple, a competitor would.