Certain kinds of marketplace content are dependent on particular features where the implementation can change with a new version of the engine, so assets need to be altered by their creators and then tested and re-reviewed as still working on the latest engine release. Although it’s frustrating sometimes (for example Runtime Mesh is still only listed as being usable on versions up to 4.16 - though there’s good reason for that) the alternative would be a lot more messy, in my opinion. If they didn’t keep track and monitor what works in which versions, you might end up buying an asset which was totally broken, and not easily knowing why.
With assets that are just meshes/textures etc. you could very easily create a project for the most recent version they’re listed as, and migrate them to a project for the current version of the engine, because you’re right, in cases like that it seems more of a formality, but it’s an easy thing to get around.