While it may seem like a logical step, it really isn't when you look at the Qun as we've seen it presented.
Recall how the members of the antaam in Da2 react when you go after the Tal Vashoth, rather than being grateful they remark that you stole their purpose. Despite this being a very dangerous task, they would have preferred to do so themselves. This we can also see in the snippets of qunari text we have in the codex: people have purpose, and they can only be at peace with themselves and the world if they follow that purpose. The dead have no use for a purpose, the dead are dead.
People are their purpose. Raising the dead to do their work for them is to rid them of their purpose, and thus by extension their selves, would not be the logical conclusion of their philosophy. It'd be a twisted mockery of it.
What point would it be to raise the dead to remove the need to work when the work itself is what is desireable? Results are important to the Qunari, but the process of getting there even more so.
The difference between using a machine to assist and the walking dead to assist (aside from how disturbing it would be) is that the qunari who'se purpose it is to do a certain task can learn to operate a machine to do his/her work more efficiently, whereas only a sareebas could command the walking dead. It would mean someone else having to do your work for you.
So even if the Qunari changed to become accepting of magic (unlikely to happen, since it's tied to the same belief outlined above) they still would only raise the dead for labour as a last resort.
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Besides... raising the dead for labour would be an insanely dangerous endavour. Looking past the sanitary concerns, the walking dead are still likely to frighten and upset people. Their bodies may be empty shells, but they used to be people they knew after all! And in the wake of those emotions come the demons of rage and terror.
And the pure hubris of it all might just attract demons of pride.
Nothing you want around those bodies you've yet to raise in other words.... or your mages for that matter.
Plus... have we ever seen undead used for anything other than the purely mindlessly destructive?