It's because they sell their hardware, software and services to consumers, not to ad buyers. It has nothing to do with their cash hoard (Apple is perfectly happy to maximize their profits regardless of cash on hand).

It is nice that Tim Cook is pro consumer privacy and that gets baked into the company culture, but Apple is also financially motivated to continue this stance (it's a major differentiating factor and marketing point).

It's because their business model involves making stuff, and selling that stuff to people for a profit.