There's a limit for file size on the Usenet

No, there is no limit on the file size on usenet. There's a limit on the individual article size, but larger files just require more articles.

The reason why files were split on usenet was completion and corruption, and probably also media size originally. Say you need to post a 700MB file to alt.binaries.erotica.grannies.diapers, then you could just split those 700MB into 477867 articles of 1.5kB each, but if a single article is then corrupted or dropped, then nobody can get the file. If you split the 700MB into 35 files of 20MB each, and each 20MB file into 13654 articles, then a dropped article only corrupts a single file. Add to that, that completion issues often occured (or is it occurs? it's been a long while since I got my Linux iso files from usenet) close to each other. So there might be a bunch of corruption in a single file, but everything else is fine. This is useful if your main provider was your ISPs complimentary usenet server, and you only got the rest from a pay by download service.

About the media comment earlier, I can't be sure. I wasn't around in the early days, but I know that the 700MB file size for movies came from the limitations of CDs. Splitting files can quite possibly stem from some similar restrictions on a removable media.

You can thank WinRar for powering the entire sharing scene for decades

And the saints behind winrar for only bugging you to pay. TBH first time installing 7z on a new windows install, instead of winrar, felt a bit sad.