Simply because you Can't spend 15 minutes or more on a mattress.
When shopping for a mattress online, you cannot physically test it out before making a purchase. Your only option is to rely on the information provided in the product description, such as the type of mattress, firmness level, and materials used. However, when shopping in a store, you have the opportunity to lay down on each mattress and take your time making a decision. To ensure you choose the best option, it is recommended that you spend at least 15 minutes lying down in your typical sleeping position for each mattress you are seriously considering.