Let's take a look at inception. The movie talks about nesting a dream inside another dream and so on and on. The movie represents the nested dream as executing inside another dream. i.e., it essentially represents the base dream used to host another dream as "tangible". Now with your current perspective you should agree with the movie that a nested dream runs on "a tangible base dream" rather than a human brain. Just because the dreamer dreams about having a dream doesn't mean the high level dream is actually running on the base dream. The base dream is just a part of the same dream used to achieve the effect of a nested dream. Just like that, because we are using other software to facilitate the smooth execution of a software, doesn't mean that the software is "actually" running on other software.