Apple could do it, but it's a very expensive bet.
TSMC spent $9.4 billion on their latest 300mm fab, and it will be running pretty much 24/7/365 for many, many years. And if Apple broke ground tomorrow, it's still likely to be 3 years before the fab is fully built, equipped, staffed, qualified, and running at full speed. Unless Apple is sure it can get chip volume high enough to achieve real cost savings, it's probably not worth doing.
And by then the industry might have started transitioning to 450mm.
Apple is one of the few companies in the world who could drop that much out of actual cash-on-hand without blinking, but it's a very risky bet with potentially a lot more risk than reward.