Why does the product page not say shipping would be free?

The line stays away where the sum could be wrong or does not apply: when a coupon is in the cart and your free shipping counts the cart after discounts, since a percentage coupon takes part of whatever is added; when your Free shipping method needs a minimum amount and a coupon, and the shopper has no such coupon yet; once free shipping is already unlocked; for a virtual product; for a product or variation that is out of stock or cannot be bought right now; for grouped and external products; for a variable product with more than 100 variations; and on screen sizes where you have hidden the bar under Display. On a variable product it waits until a variation is chosen. It works from the product's own price, so a discount another plugin applies in the cart is not taken into account.