That’s because Amazon is operating as a retail pharmacy that then ships your medication, not as a mail-order pharmacy. This is because most plans have exclusive contracts where you’re required to use a specific mail order pharmacy, so operating as a “retail” pharmacy allows Amazon to accept many more insurance plans.
My insurance actually requires 90-day supplies for maintenance drugs, and I can get them filled at a retail pharmacy that way (they do require CVS). But even with insurance, it’s still cheaper for me to get it at Costco or a grocery store using GoodRX. Amazon has no option, even without insurance, to request a 90 day supply.