not sure why nintendo didnt include it in super mario bros.
well... actually... i do. the reason is this way they sell 2 games instead of one, and not including a story mode in this saves them more money. to get to the point though, yeah, this should have been with its own story mode.
http://www.vgchartz.com/article/259722/super-mario-maker-features-100-levels-created-by-nintendo/