What's the answer to the mixed attacker problem?
In order to have decent offensive stats, they need to either be slow, frail, or both. Only exceptions being legendaries, pseudos, and megas who have larger BST's.
And even if a mixed attacker has good stats, there's still the problem of investment. You can't invest fully in both attack and special attack without sacrificing speed or bulk, so most mixed attackers can't even viably run mixed sets and just have the option to go either physical or special.
Pic related, a key example of mixed attacker failure. It's a shillmon that GF wants to succeed and even it can't keep up.