Like you, I just figured it out for myself as a child that santa wasn't real. In fact, I always knew he wasn't real even when i was like a little kid. I just lied to my parents about believing in him up until i was like 10 to get more presents for christmas. lol. What can i say? I was a smart kid back then. lol. Same reason why i went along with the whole tooth fairy myth, as i knew my parents would give me money for my teeth. lol.
All kidding aside though, the childhood myths we tell to our kids can be an important lesson to our children as well when you stop and think about it. As nostalgia critic said in that video link i gave you, he points out how when kids discover that santa isn't real that kids will undoubtedly be upset about it, but it'll also teach them an important lesson. It'll teach them that sometimes in life that no matter how much you believe in something that sometimes things won't always turn out the way they want it to be in life, yet that doesn't mean the world will blow up or end just because it doesn't work out.
It teaches kids how to adapt when things like that happen. It teaches them that in life not everything will always work out, so in a way, you're teaching your kids to be more prepared for the real world someday.
Plus, the world is already filled with misery, so why not let the kids believe in a world where santa rewards the good kids of the world for a little while. sure it may not be true, but it's like some hubbers here said, it'll help spark their imaginations, and help them grow up to be better individuals.