Some might for that reason, claiming some of them as their own for the ego hype, but I don't see that being a big reason.
Others will have their own little sites or pages setup for collecting click/ad revenue or just to sell the mods outright, usually to an audience of newbies that don't know that the original free versions exist.
Another is people that want to speedrun setting up the "competition" to this site for whatever reasons they have (not getting into that here, but the theme of this thread is likely very relevent).
This sometimes gets coupled with a "you can claim your mods by signing up and messaging us" - which increases their numbers and makes things seem more legitimate when it's not.
Then there's malware infectors grabbing files to redistribute with their BS included to scrape other types of data, grabbing large amounts of differing mods to redistribute elsewhere to reach a large number of people.
There's a ton of other reasons/theories/ideas of why, but those are the ones that come to mind first.