Because it does not work, it is called game design, you design to a system because it fits into the rest of the combat you've designed. You don't arbitrary throw it stuff because well you know you should. Lock on would NOT work with DmC, it would not solve the issues everyone seems to be whining about constantly. You are always holding Angel Trigger and Devil Trigger, it would not be something that is natural to be holding lock on + angel trigger + demon trigger. The game's AI would have to be completely redesigned if you were to legitimately design it around one on one lock on combat, because it would break the game if it worked as some people would wish it did.
Dante dominates enemies one by one, the only challenge during DmC's combat comes from managing environmental hazards, and mixed crowds, because you have to utilize various different tactics in order to keep your combo going. Once you get to the last enemy, the game does lock on to the final target, and now you can do your stylish combo finish in true DmC style.
I've said it before, here's what would happen if I gave lock on but kept the exact same AI rules, the lock on would say (sorry cannot target, enemy using disruptor ray to prevent demon pull, or I would make it so the enemy would 100% repel your demon pull.)