Blame copyright laws.

Any all-in-one solution is going to be considered illegal at face value if it is obviously designed to circumvent copyright.

But if you split all the pieces of the solution up into separate pieces of software that can't do much illegal by themselves, you can always claim plausible deniability as the maintainer.

After all, you aren't going to be downloading torrents with Sonarr, you aren't going to be looking up information about shows with Jackett, and you aren't going to be automating your downloads with qBittorrent. If you link them all together with APIs, though, they do everything automatically.

As for the issue demonstrated here, it's just a long-standing problem in Sonarr and Radarr that even when it detects that a torrent is bad, it just sits there holding onto a virus until you override it with a working torrent, instead of just dumping the bad data and trying again.