You should really brush up on copyright law. Everything he created from scratch in this map as well as the map itself are his digital intellectual property. As such, he has not stated that any of his created property is designated as being in the public domain, therefore it cannot be freely redistributed on any other sites and the items he created within the map cannot be used in another map without his permission. It's people with attitudes like yours that eventually cause modders to stop making mods.
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At the bottom of the Official ModHub website. Now Go touch Grass
You obviously have no understanding of what open source means. If the game was open source, the entirety of its code would be available for users to make changes to. Open Office is one example of this. Most of the code in FS25 is locked and can not be modified or even viewed easily by users. While you can view all of the code for a mod, these fall under copyright. I am assuming you are under the age of 21 based on your comments.