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Because t's too hard
Most pokemon only use five of their stats anyways, HP, Both defenses, Speed, which is less important on saturable slower mons, and one of the two attacks. Mixed physical/special builds are avoided because EVs would have to be split between both the mon's attack stats, whereas choosing one or the other allows you to pump the max amount of EV into your offense and still have a bunch left over
Another reason, the destiny knot. The destiny knot carries over five stats from the parents. This is useful in te case of, say, having two parents with One having perfect defenses, with the other having perfect Hp, speed, and the attack you chose, breeding them, and hoping that the five stats chosen, chosen from among the parent's twelve total, were the five perfect ones.
