When downloading Minecraft resources from third-party sites, the chances are high that the file is an old release. That means it likely contains bugs, or features that are missing from newer versions. When they create a page containing a download to a mod, it’s never updated again.
Minecraft developers are able to receive an income from creating mods, whether it be through the CurseForge rewards program, Patreon, advertising or more. Downloading files from reposting websites strips developers of potential income, as well as incentives to create more content in the future.