The problem with that argument is that the sim ships with 30,000+ airports that are auto-generated as part of the base price. These airports are free, and enough for you to get going, but they are generic and not realistic.
Third-party payware airports are highly specialized. They can take tens if not hundreds of man-hours to cover research, planning, modelling, testing, marketing & publishing. These airports are designed to appeal to people who like hyper-realism on the ground, which is why they have value.
I can’t imagine you, or any other person would be wanting to buy 10,000+ let alone the ~30,000+ that are in the sim.
Honestly, $15 isn’t that unrealistic. There is the potential for significant cost when developing an airport. If the developer(s) is an organisation with staff, they have to pay wages, software licences, other development fees potentially including IP licences, research and even publishing costs, not to mention web fees if they have their own website, forums, domain, email, support system etc.
All of these costs add up, and they need to be paid for, and while an airport may be relatively cheap to develop, think about the complexity and cost involved in developing realistic airliners (Q3/400, ATR, 737, 757, 767, 787 etc).
Developers need to ensure they get enough money from sales to cover their development costs, and to help fund future development, plus their other services.