> Emergency measures being valid for the duration of the emergency would be pretty straightforward.
Deciding when the emergency is over is less straightforward than a fixed time period. A government that wanted to extend the measures indefinitely would just claim there's still an emergency.
But the cooperation of legislators is required because of the fixed deadlines. If the government would be allowed the right to extend the provisions on their own, they probably would try to do that in perpetuity. With a legislative branch that actually cares about civil rights, the former is the much better option.
Deciding when the emergency is over is less straightforward than a fixed time period. A government that wanted to extend the measures indefinitely would just claim there's still an emergency.