The reason for this is that a large percentage of builders don't post their builds on the forum.
It's hard for them to be known if only a few people have seen their builds.
This.
Add-On Makers recieve so much 'fame' simply because their stuff is there to be viewed by lots of people.
When someone makes an Add-On, they generally tend to upload it to the internet, and allow others to download it and use it. So, people will remember that as it's on the Internet, on a Forum.
Most Builds however don't end up being posted in the Gallery, so people don't see them. There certainly are loads of brilliant builders out there, but they're just harder to spot, and they don't come so massively public with it. And, as Ephi said, people build something, then toss it out of sight and build something new. You could tell people that "XxX has an amazing YyY on their server, go and see it!" but that person could have put it away and begun building anew. So, even when the builds are kept on servers, and don't show it on the Internet, they get missed, so people don't get to see others great builds.
Simply put.
Add-Ons: Generally massively Public. Creators get credited by a lot of people.
Builds: Generally Privately kept upon servers. Creators get credited by the few people who saw it.
