Fandoms tend to always be easy to spot when if you keep your eyes out for them, or just have reason to notice them. Just try to notice all the anime, or pokemon, profiles on steam. The furries and the bronies tend to stand out more due to them being more "different" then other fandoms, but don't really occupy a significant part of the tf2 community as a whole. As to why they are there at all is most likely just due to the nature of the internet.