Apple typically applies vertical integration on customer facing elements. Generally, that means hardware integration via software.
By acquiring TSMC (or any other specialized downstream component manufacturer), Apple would be taking on huge oversight responsibilities for... what exactly? What gains are you implying that make this such an obvious decision? How would such an acquisition benefit the end customer?
Considering that Apple already has a design team developing their chips, I really can't think of anything to be gained.
As someone who worked on design teams at places that owned the fab and placed that don't, owning the fab is better. The result is much better when the designers can influence the process.