Yes, you can plant a seed in some anonymous dirt, just give it water, and you will get some bud. So why don´t experienced growers just do that?
Because if you use a nutritious soil mix and give it some ferts you will get a much larger yield of better bud. Nothing new here - farmers have known this for millenia.
When growing MJ, as with most human activities, what you get out is directly proportional to the time, effort and money that you are prepared to put in.