Why I can't export Paragon Assets

About Paragon Assets.
It’s amazing that we can use all these assets for free, so thank you!
But I really can’t understand why I cant migrate one single asset to 4.18? I could understand if I were migrating a class code for example, but it’s impossible to migrate anything.
A mesh, a texture, anything…

I can export some meshes in FBX, but some others I can’t… Even some really simple textures are blocked to export.
Why are all these assets blocked in that way?