It's legacy, white spaces weren't allowed as characters on most FTP software, which is how the warez scene shares it's releases. It used to be underscores, but dots are closer to a white space regarding separation (space wise), so most release groups use dots nowadays.
Generally, a white space as a character in filenames and directories is "frowned upon" in many operating systems, Windows included (somewhat). It makes writing scripts and software more comlicated because it's used as as a separator for giving command line/terminal options to commands and binaries (programs).
it goes way back before ftp.. i believe its because the original operating systems filesystems/namespacing could not handle the space character at all. so all files lacked spaces in their names. but only for like the first 30 years
Yes, you're correct, it goes much further back than FTP, all the way down to UNIX I believe. The problem was commands and parameters (options) which use a white space to seperate between them. So, filenames and directories were't allowed to have white spaces in them.