Why is the top step of a stair a different thickness than the other treads?
The top step is not a tread. It's flooring.  You buy the stair nose as part of your flooring purchase, not the stairway build.  Most manufacturers of hardwood will provide a matching stair nose, but you may have to order it.  



Note that the stair nose is a groove.  You can also purchase a double-tongue that will slip in that groove converting it to a tongue should you need to.