Getting a large number of mages together doesn't suddenly make them an army. Most of them have no battlefield experience or knowledge of basic military tactics. It's an extremely well (magicaly) armed mob, but a mob non the less.
Most battlefield and military tactics are made by people who have nothing but their own physical bodies, physical handheld weapons and physical projectile weapons to work with, and have to finagle how to use these to their advantage against other people with the same types of weapons at their disposal?
In Thedas, most mages have enough firepower destroy an entire village in one go. To pretty much rain fireballs, ice storms, death clouds, physically and mentally weakening spells, etc. at large groups of people from a distance, before they even get close enough to touch them. And they have so many diverse spells with different abilities; elemental blasts, defensive
Keep in mind that even those mages who did take part in recent conflicts (like the 5th Blight) did so under the command of people who understood war and how to best use mages. They were acting like support; throwing fireballs and casting spells behind solid lines of regular troops. The trick to their use is to make sure they are free to focus on using their magic without having to worry that somebody will charge in to chop their heads off.
And if there were more than seven or ten mages, they could have thrown enough firepower to wiped out the opposing army before they got close enough to chop their heads off. Individual mages have enough strength to flatten a village, remember? Imagine a whole village-full of mages, all attacking and focusing their strengths together. Or, you know, using the defensive and healing spells on themselves and each other.
Did we actually play the same game? Because I played as a mage in DA:O and by less than halfway through the game, I no longer needed party members or meat shields to defend me. (In fact, one of the most common praises/complaints I heard about DA:O mages was how much sooner and more easily you could solo through the game than the other classes.) I could cast powerful AoE and crowd control spells and wipe out, or at least severely weaken huge crowds of enemies before they got close enough to do damage. And for those few who still survived the onslaught, I cast health/mana drain force fields around myself, so they became a lot weaker and a LOT easier to pick off in hand-to-hand combat. And that's before I learned blood magic and the glorious Blood Wound...
This time, they don't have such protection available nor an experienced commander to lead them.
And of course, it'll ALWAYS be like that. They won't learn, adapt to their new situation, use their environment to their advantage to make up for not having the Big, Strong, Clever, Experienced, DESPERATELY NEEDED army general to protect them. Like guilla warfare: attacking from a distance, from hiding, from elevated or fortified positions. And the suppor they have now, between these two games, is the amount of support they will ALWAYS have. They can NEVER gain new allies. It's completely impossible for an experienced commander to join their side, or even find an experienced commander among them, etc.
Nope, they'll always be inexperienced, unsupported children that are incapable of learning, adapting, or gaining new allies, and will just sit around sucking their thumbs like two-year-olds waiting for the Big Strong Soldiers to run up and lob their heads off.