I am DMing 2 games at the moment and playing in 1. I was playing in 2 not so long ago.
In none of these games do I see PCs doing OAs often.
Though this doesn't really track what we care about, does it? It isn't just how many OAs the party takes. Isn't it also how many actions the enemies don't take in order to avoid OAs?
I mean, I understand your point. Monsters can act tactically to minimize the disruption a defender can cause them. No Defender is so good as to shut down all enemies from getting to the squishier PCs.
On the other hand, PCs can act tactically and maximize how effective their defender can be. There
are many situations when a fighter can pin enemies in a tight spot, or allies can get to safer places behind the fighter, or often produce their own elements to alter the battlefield and keep their casters safe.
It won't happen all the time, but a defender should usually be able to get the attention of a couple enemies and keep them occupied. Any claim they can't do that - that all monsters are so cunning and so mobile that they can always slip away to destroy the sorcerer - I just can't agree with. Nor can I agree with claims that the defender will keep the sorcerer from every getting attacked, of course. But I think you are, in an effort to get across your point, vastly under-representing what a fighter is capable of.
And maybe that is the experience in your games. But the games I have played, with both a tactical DM and tactical players, have not matched the scenarios you describe.
