Arc isn’t doing that intentionally.
There’s a method of uploading the game to itch.io that makes people using the launcher able to update the game easier.
Sometimes it may randomly decide to put the APK inside a zip. Arc has no reason to intentionally do that.
If you can’t be arsed to unzip the file. Just check Patreon or Subscribestar and use the public release links from there. They’re the same version that itch.io gets.