The issue only arises when you buy non-refundable tickets. If you choose to do so, that's your decision and you enter the contract with eyes wide open.

If you purchase a refundable ticket for someone who can't travel, all you do is cancel it, take your refund and buy a new ticket for the new pax.

Why is it that way? Because it is what consumers willingly agree to in order to save money overall even if they lose money once in a while.