Technically they do.
I dont know how im going to explain it to you.
Its the profession of war.
People die.
Also, whats with this notion that the grey wardens can predict the future and know when a blight is coming?
Its not running on a timer, its dependant on events to fall in place.
They need to always be ready for the blights, not just sit back and then drink a vial when the blight occurs, they wouldnt be ready, or used to having the darkspawn poison in them.
And who would lead it?
If there arent any grey wardens between the blights, none at all, oh boy, Murphys Law trembles with joy.
It also appears that grey wardens are somewhat of diplomatic envoys during peacetime, and you can NOT disregard how important that is.