Because US customary units are a pain? :D Judging from shopping trips with housemates, I imagine most people don't look at those things, so putting more effort

Because US customary units are a pain? :D Judging from shopping trips with housemates, I imagine most people don't look at those things, so putting more effort into that feature isn't worth it. But then again, Amazon's search is also notoriously bad, so maybe they simply don't have good metadata for products, and they're all derived in an ad-hoc way.