This is mainly for security. Since jars are "executable", a user could upload a malicious jar which would be malware when ran outside of Minecraft. By putting it in a zip, it prevents users from just clicking and running those files right from the download.

We don't manually review each and every file, so this tries to help reduce the risk of malware being shared.

That said, I understand the annoyance, since mods are jars. I'll talk with Cyp about it and see if we can look at reconsidering this.

It was always like this. just upload your .jar file to .zip

Just add the .jar to a zip folder, problem solved!