Because you’re treating your purchases as if they’re rentals, which they’re not. The merchant has to take back your used stock, give you all your money back, and the a) sell it as an open-box product, or more likely b) throw them away. You are putting them at a loss because you want to use Amazon like a Blockbuster when that’s not what they’re for. In the end, everyone ends up paying for this because the retailer has to adjust their prices to factor in these losses.
Also, you can and will get banned from Amazon and other retailers if you keep doing this. Abuse of the return system is monitored and they will cut you off as a liability. Then no more IEMs for you. Anything more than a 10% return rate is suspicious and will attract the wrong kind of attention.
The technical term for this kind of damage is permafucked.