Bitcoins are a real currency with value backed by the interest in silk road products. Why isn't one bitcoin fixed to dollar or euro by some conversion factor?

Bitcoins are a real currency with value backed by the interest in silk road products. Why isn't one bitcoin fixed to dollar or euro by some conversion factor? That would remove the question of the value of a bitcoin. There are lot of currencies that are permanently fixed to an other. The novel thing is the theory that you can cryptographically make anonymous transactions unique. It doesn't imply being a separate currency. Or am I missing something?