"Language Change as a Source of Word Order Correlations", by Brady Clark, Matthew Goldrick, and Kenneth Konopka, is among the many sources dating back to Greenberg (1966) stating that language learners prefer a consistent branching order over an inconsistent one. For example, OVAux (object-main verb-auxiliary verb) constituent order and its opposite (AuxVO) are more common than "brace" orders such as AuxOV (German).
Consistent branching with the verb phrase as the head of the sentence would predict VSO or VOS. Yet SVO is about as common as the apparently more consistent SOV, despite nouns coming on both sides of the verb. SVO looks superficially like a "brace" order rather than consistently left-branching or right-branching.
So what makes the subject an exception to this consistency? Or in SVO, is the noun the head of the sentence in the sense that Aux is the head of the verb and the verb the head of the object?