A lot of companies exist that make various components. BUT few with the quality and volume of Samsung. Apple is often supply-side constrained. So they could

A lot of companies exist that make various components. BUT few with the quality and volume of Samsung. Apple is often supply-side constrained. So they could cut Samsung out the mix, but this just makes Apple more vulnerable to supply shortages. Furthermore, by selecting their way out of one of the biggest screen, CPU, and memory manufacturers in the world and probably having to fund new factories for smaller manufacturers -- this volume is open up to other companies. The end result is cheaper components for Samsung mobile, HTC, Sony, LG, etc...