system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why some files are archived into like 40 .rar files instead of just one.
gpt
Intro that explains how pirated things work. If you arent interested, you can skip to below the line.

It has to do with the "Scene". The "Scene" is basically where everything pirated originates from. It is a collection of servers(often called "topsites") and groups of people. Those groups crack games, pirate films, music, etc and then upload them to a topsite(or more). The result of their effort is called a "release" and it usually named like this

Battleship.DVDRip.XviD-DoNE

First goes the name of the material(movie/game/whatever), then the type(dvdrip xvid) and then the name of the group who made it(DoNE). The scene has specific rules regarding the way you can name a release, how you encode it, the file size, etc. From the topsite, the release leaks to other high quality private trackers and dc++ servers(the main uploaders of those sites usually have "scene access") and eventually reaches public trackers like thepiratebay. On their way to the public tracker, those releases are often renamed, unrared and sometimes even reencoded(which lowers the quality of the film).

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The Scene uses ftp servers. Ftp doesnt the same kinda of mechanisms that bittorrent has to secure file integrity, therefore the scene has some rules about how your release should be. Also remember that the groups are often racing against time. They want to release their stuff before other groups so they get the credit. Only the first group "wins" and gets the credit. So as long as there isnt anything wrong with the release(it isnt nuked), other groups cant release the same thing(though they can release it as internal).

The rules say that your release should have a hash file(.sfv) that makes it easier to check that your release was uploaded without getting corrupted. Files can often get corrupted, especially when we are talking about really fast transfers to multiple servers. The multiple rar rule comes in useful in the case your release got somehow corrupted, since instead of having to reupload the entirety of your release, you only need to reupload the corrupted individual rar file.

If a warez you found is in a single rar, then it probably isnt "scene" or most likely, it was a scene release repackaged by someone. Personally i strongly release to only download scene releases, preferably from a private tracker since this greatly increases your safety and decreases the chance of downloading a malware.

TLDR : Multiple rars exist so it can be easier to fix a corrupted release.