Muscle tissue has the sole purposes of enabling movement of the body and organs, fat tissue is mostly only for energy storage. So neither of those two tissues really qualify as a protective tissue and if we start thinking evolution it makes little sense for an individual to have increased reproduction rates because of two ridiculously energy cost intensive tissues around the skull which already protects the brain.
Tissues that are fit for acting as protective Barriers are bone, skin, and mucous membranes and those are already pretty solidly implemented into our heads at this point. Anything beyond that would probably just drive the cost-effect balance into negative and would decrease our fitness as individuals, thus leaving us more susceptible to natural selection which would lead to a relatively fast extinction of such a muscle-fat-head human.
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