If those numbers are true, why does Apple bother making stuff? Their $80 billion in the bank should, according to your figures, make them $8 billion a quarter? Actual reported profit is $6.6 billion, so they must be losing over a billion on production and sales of stuff each quarter.
There are short-term securities that earn 10% in 3 months? How do I get in on that?
I'll leave the search for 10% 3-month yield to the parent, but note that the math still works for much smaller yields. Rolling e.g. 12-month US treasuries will yield around .1%, which is roughly $40m annually on the hypothetical $10B. More or less risk free and with excellent margins. This is meaningful even to a company as large as Apple.
No doubt this behavior began before the Fed started their zero interest rate policy. Come to think of it, Apple was in business at the end of the Carter Administration when interest rates hit and exceeded 20% (!!!)....
By delaying payments Apple can use the cash for other short-term investment opportunities. In Apple's case, their accounts payable is probably in the billions. As an example: If Apple has a $10 billion accounts payable and instead of paying it right away they waited for 3 months and invested that $10 billion on short-term securities with a 10% return, Apple would earn $1 billion in that 3-month period vs. ZERO if they paid their liabilities right away.