""Why not less complex?"
Because that's not what the theory of evolution requires"
You have put your foot in it big time. There is no "upward direction" to evolution, only in finding more fitness. Consider parasites.
I take it the "amoeba to man" scenario is then incompatible with evolutionary teaching and thought. Matter simply develops and self-organizes any way it might, with evolution simply meaning "change." The notion of "complexity" is an arbitrary one, since the definition might imply elements of design, which in turn is inimical to naturalist assumptions.