Because if I'm Apple and sitting on billions of dollars I get to buy out my supply chain and I pay 0 tax for all components and transitions in the chain - and your random manufacturer X has to pay Qualcomm and gets taxed, who pays TSMC and gets taxed, who pay their suppliers and get taxed, etc. Then multiply this by every component at every step. This is a terrible incentive system for breeding huge vertically integrated monopolies even faster than we currently do.