I assume that would be done after bringing a drug to market, otherwise drugs could easily be delayed 10+ years if they're shelf-stable.
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?
This needs regulatory work. And this bring very little returns, so your regulatory resources are better spent somewhere else. There are other incentives in place.
It's very likely it is not favorable to companies to look into this, just as it was revealed that the EpiPen expiration date was not really true.
"able to calculate the longevity of chemicals, food, and other products without having to wait the actual time period"
What kind of error rate are you willing to accept in models of pharmaceutical shelf stability? Bearing in mind that errors potentially translate into deaths, probably disproportionately of the less privileged among us?
I'm not anywhere near knowledgeable on pharma, but I worked on an FDA regulated medical device. If we made a claim, we had to test it. Knowing that e.g. every component in the machine was rated safe between temperatures X and Y did not mean that we didn't have to perform real environmental testing.