Nobody said they need to find the absolute last day the drug expires before they can get it to market.

If it's a new drug and they can only say for sure that it lasts 6 months then put that on the label and ship it out. But assuming the drug will be out for 10+ years they should be continually testing the expiration date and updating as they've had more time to make that determination.

But like I said initially (and as you nicely pointed out) that costs money so why bother?