What are you on?
A greatsword is ideal for parrying - It's securely held in both hands so it can be maneuvered faster than a Dagger. It has greater mass, meaning that when it takes a blow, less of the impact is transferred to the user (You'd break your arm trying to block with a dagger), It has much greater leverage for re-directing the momentum of an incoming attack, and it has a much larger surface area to absorb the blow. The difference between trying to block with a Greatsword vs. Block with a Dagger is like the difference between trying to hit a Tennis Ball with a Racket vs. a Chopstick.
A greatsword was not historically used as a fencing weapon and the idea that you can move one around faster than a dagger is ridiculous. Moving one hand with barely any weight is faster than moving two hands syncronously with a somewhat greater weight in them. No, greatswords were not even close to as heavy as the game oddly (and very unrealistically) depicts them, but they still weighed a good deal more than a dagger. The advantage of the greatsword was greater reach and greater impact, which meant they'd have an easier time knocking aside enemy weapons, which effectively would leave the enemy soldier more or less unprotected.
Not that I wouldn't rather try to "block" a weapon swing with a greatsword than a gladius, if push came to shove and I absolutely had to do that, but trying to block a mace or hammer swing with either weapon is crazy. I weigh roughly 95 kg and if I put that into a swing with a blunt weapon then it will overpower your wrists easily, no matter what weapon you're blocking with. It doesn't really matter how strong you are because your wrists were never made to tolerate that much force.
If you want to stop the attack from knocking you off your feet then you need to be proactive and stop it before it gets momentum or redirect the direction of the force. Or use a shield and let your entire arm take the force, which might still knock you off balance but at least isn't going to disarm you or break any bones. God help you if you actually tried to use a sword to block a 20+ lb hammer-blow from someone strong enough to wield such a hammer.
Trying to hide behind a static block the way the game lets you do it is just a road to suicide, so complaining that it's unrealistically inefficient doesn't really make much sense in my book. Weapon-based blocks really should have been timed counter-swings that intercept enemy blocks if timed right and do little if done wrong. Parrying with a hammer would seriously stagger the enemy, for obvious reasons, but would require spot-on timing and leave you wide open if mistimed.
Of course weapons with a long haft would also allow you to use a staff grip and block with the haft, and that option really should've been in the game as well. Greatswords occasionally had unsharpened rosettos for a reason. That reason wasn't just that it looked mean but that a wielder could safely move his hand closer to the center of the blade for improved accuracy. And speaking of details, there really should've been more focus on the footwork as well.
A small sidestep at the right time is all it takes to make a hammer-wielder thunder past you with his giant overhead swing of unstoppable momentum, leaving him entirely open to whatever form of retaliation you feel like handing out. And since the hammer-wielder can't stop his momentum either, he essentially commits to the action seconds in advance of it landing, which is precisely why it wasn't ever a good idea to use giant hammers and make huge power attacks. Pretty much any foe would see the swing coming, calmly step out of the way, and finally counter-attack while you're recovering from a heavy swing against thin air.