Apple only has 270 stores in the US + their website. The vast majority of their phones are sold primarily through carriers in the US. Grain of salt and all, old
market research data suggests 10% of iPhones are sold through their stores. Granted, their percentages have probably grown since 2017, but not to the extent of majority. The combination of brick and mortar carrier/3rd party retail locations, their associated websites, and the beast that shall not be named (Am z n)...
Anecdotally, I've been buying Apple products for more than a decade and never purchased from an Apple store or Apple online. Been in the stores to look, to test drive. Buying inevitably is less expensive from a 3rd party retailer like BB, Microcenter, B&H, etc. I imagine a lot of people do the same.