Why waste resources training mundanes to do something that mages can do but better?
Imagine if Tessa and Marius were mages. One casts spells to cancel out the targets magic while the other casts spells to incapacitate or kill.
Seems vastly superior to how they currently kill mages doesn't it?
Tessa is not a formally trained mage killer -- the Tevinter term is perrepatae -- she is just Marius's tag-along rogue buddy. As of right now, we don't know how they got together. Marius was a slave, as Fenris was. I'm going to assume that Marius was trained with those skills to act as his master's body guard, just like Fenris, but without the fancy lyrium. I'd imagine that if Marius's master were attacked, that they would work together to subdue the threat -- a mage + warrior combo is a powerful one.
If we're looking at it from a strictly political view, having the ability to defeat your opponents in such a way would send a powerful message. It shows that you have resources to "waste," shows off your own skill with magical combat, and is a demonstration of what you are capable of.
Aside from those above, the writers of Dragon Age content haven't really attempted to have a great deal of mage physicality, mages who engage in physical combat while casting spells, aside from the rare specializations of Arcane Warrior (shown in DAO to be a lost art), and Knight Enchanter (said to be rare). I don't really count the staff flourishes of DA2 and DAI, as that is a very gamey element designed to act as combat filler when the player mage is not casting.