Because a lot of things happened that was near impossible to predict.
Most of the scouts sent out were killed, and those who did make it back talked of a large horde, and many of them, like the one we can talk to, are driven mad by fear or by the taint.
Exactly. They didn't have all the information and actually both Cailan and Duncan said that it would be better to await Orlesian reinforcements. Loghain protested the "fool notion that we need the Orlesians to defend ourselves". He was wrong. The Wardens did know the real danger. If Loghain didn't believe them, it was his fault and his fault alone. We know that paying attention to the Wardens was the right strategical option in Ostagar.
The tower was taken almost as soon as the battle started.
True, that and the bacon being lit late wasn't his fault.
Because Cailan chose to charge out of the choke point and allowed himself and his men to get surrounded.
That looked more like Hollywood tactics to me rather than Cailain's fault. I mean, they even used fire arrows! And that was something prepared beforehand, not just Cailain doing a Leeroy Jenkins.
The horde was still pouring out of the wilds at the time the beacon is lit, thus rendering the hammer/anvil strategy pretty useless, and if he had charged all that would've gotten accomplished was Loghain getting himself and his men surrounded as well.
Ah, but as you see, I didn't mention his retreat at all in my criticism. I don't criticize him for saving half of the army, I'm criticizing him for losing the other half. Because you have admitted too that the hammer/anvil strategy was pretty useless. It was his strategy. Because when you are defending a fortified position, there are more options. Longer, more tiresome and less decisive options, but also much safer. Like entrenching yourself and wait for reinforcements. But as Loghain made it very clear, he didn't want Orlesian reinforcements.
In fact, I interpreted the whole battle as Loghain wanting to win a decisive victory to show that Ferelden didn't need Orlesian help. It couldn't be Cailan wanting to earn glory, because he was actually the one suggesting to wait for the Orlesians.
Also, if you listen to rumors from Bodhan or the tavern keepers, Loghain effectively kicks the entire bannorn's collective butt in the civil war as the game progresses.
Half of the Bannorn was already suffering the Blight and the other powers that could stop him were incapacitated (Cousland and Eamon). Meanwhile, the Wardens were already building their own army and no matter what, Loghain losed the civil war.