Why do you start at Bronze?

Why is the lowest ranking tier Bronze? Shouldn't you start as unranked and, instead of placements, have to work your way into bronze? Just deciding one day you want to start playing ranked should not place you on par with people who are trying their best to work their way into Silver and, in many cases, have put several several games into their efforts.

I am not complaining as I have never had an issue climbing out of Bronze, but I feel like an "unranked" tier instead of placements would be a lot better for the community and game itself.