Mods are definitely a part of the success, but yeah, it's likely not the biggest reason it's so popular. Their games are popular and console mods didn't even exist until 2016 and I don't doubt you'd fine people who loved the games regardless on consoles prior to that.

Bethesda, back in 2015, during the Steam Paid Mods fiasco, released a quote stating that "Only 8% of the Skyrim audience has ever used a mod." And some time before either Legendary Edition or Special Edition released, I had done the maths on the number of copies sold of Skyrim and how many unique downloads the Unofficial Skyrim Patch had. Seeing as Nexus is the only official way to get the patch, I figured the number is decent enough and I got around 10-12%. If you're going to use mods, there is no reason not to use it. If people really think that mods, at the time, were so widespread, then you'd have to believe that nearly 90% of users who modded didn't use the Unofficial Patch.