It's not about "accomplishing" something that couldn't be done with a database. It's about making these items tradeable on a platform that doesn't belong to a single entity, which is often the original creator of the item you want to sell. As good as the Steam marketplace might be for some people, every single sale pays a tax to Valve, and the terms could change at any moment with no warning. The changes could be devastating for the value of your collectibles that you might have paid thousands of dollars for. This could not happen on any decentralized system. It could be something else that isn't NFTs but it would absolutely have to be decentralized. Anything centralized that "accomplishes the same thing" doesn't really accomplish the same thing.

It's worth noting that this sort of market control would never be considered ok on any other market. Can you imagine a car manufacturer requiring every sale to go through them? Would you accept paying them a cut when you resell your car? Would you accept having to go through them even to transfer ownership of the car to a family member? If a car manufacturer tried to enforce such terms on a sale they would be called out for it and it would most likely be ruled to be unlawful. But nobody questions the implications of the same exact situation in a digital marketplace.