Outsource the easy things so you can focus on the hard things - custom silicon provides a lot more unique value for Apple customers than commodity cloud storage.
It gives Apple flexibility and avoids secondary technical and real estate type debts.
It makes sense for Amazon, Google, and Microsoft to build out datacenters because they're both using them for their core business and selling compute and storage resources to third-parties.
Unless Apple intends to get into the cloud computing business, eating the cost of tooling up a datacenter puts a big construction and maintenance cost on the "Liabilities" side of the T-sheet that they can avoid via a smaller rental cost.
Now, if I had a fleet of cars, the calculus may change on that.
Owning and operating a datacenter is a whole kettle of fish that they don't want to get into if they don't need to. The real estate taxes (wherever they set it up) will matter, but the larger costs are likely to be in day-to-day operation, upgrading and maintaining hardware (and the entire process for that), dealing with actual natural disasters like a flood (or a bird flying into a transformer house and blowing power to N% of the datacenter, which means they need a backup generator, which means they need to test and maintain the backup generator, etc., etc.).
There's definitely a break-point where it's cheaper to pay someone else (in Apple's case, multiple someone-else's) to deal with that hassle.