[Faceman]

Plants can't walk thus they stay at the same local point all their life*.
Thus plants who have a brain go insane sooner or later.
Thus all plants with a brain became extinct throughout evolution because they became insane and committed suicide**.
Thus plants lost their brain because they couldn't walk.
Thus plants have no brain because they can't walk.
q.e.d.


* assuming they do not get move by humans interfering with nature
** e.g. leaning away from the light, closing the blossoms at all time, etc.