Probably won't take too long to read the first paragraph from Wikipedia.

Bricklink is a site for individuals/small business to buy and sell primarily individual Lego pieces, so it's important for shipping calculations to have reasonably accurate weights of all the pieces. Their weights are therefore contributed by those sellers. Although now that LEGO Group owns Bricklink, you'd think they could just slide them the numbers.

Is everyone running their own open source project?

Essentially, I suppose. I put most of my personal projects on GitHub because a) I believe in the open-source philosophy generally and b) sometimes they are helpful to others! For example, because I put SmilApple on GitHub, someone was able to adapt it to make this. And besides, it's a great way to distribute programs that you want other people to use, like my current project Chokistream, or when I made a fan-translation of a game. None of these are "serious" projects like a new framework or something, and all of them have very limited audiences, but if I'm coding them, I might as well publish them where someone else might be able to benefit from them. I also don't feel like they're important for my career, but they're important for their own sake and I would care if I lost them.