pure theorycraft-
i'd imagine that absorbing non-green light was sufficient at some point for the single-celled ancestors of plants, and they moved forward into the multicellular stages at which point other forms of mutation conferred more survivability than covering the gap would, at less risk.
you'd probably need a second non-functional form of chlorophyll to mess with, which would cost a lot in a living multicellular organism.
i mean, evolution doesn't work towards the optimal, just until it's sufficient to beat your brother.
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