Apple is a PLC, they have shareholders. The CEO's job is to increase the value of the stock for said shareholders with smart business decisions.
1. It makes no sense for the biggest company in the world to buy a football team when they have zero experience of acquiring, managing, running sports teams, and even less experience in transfers.
2. They're a tech company, if they acquire anything, they acquire smaller tech companies who have done solid R&D into products they're looking to build upon.
3. They don't need the exposure. Apple don't even advertise through anyone else but themselves, that's how big they are. At a push they would maybe be open to buy stadium naming rights but it's not worth it for them.
4. The iPhone costs around £1k, how many United fans can afford it? How quickly would you drop the iPhone if you were a Liverpool fan or any other fan for that matter? You know what really kills a stock? Poor sales, missed earnings, poor revenue growth, bad direction. All of this happens if more than half the world don't buy an expensive phone that's tethered to the United name and brand.
5. What the feck are Apple geuinely going to do with Manchester United? Like seriously? What's in it for them?6
6. Apple are a company, they have budgets, and well planned budgets at that, they don't just have £10b doing nothing somewhere.
7. R&D is expensive and so is innovation.
8. Shareholders will lose their shit if Apple take on a liability such as Manchester United.