The threat of terror gives power to the State. Power seeks more power. The State does not erode freedoms in support of safety. The State erodes freedoms to achieve more power. Terror is the excuse. It is the State's greatest tool.
The state is made up of individuals. Who are these individuals that are seeking power?
But bureaucracy is just so evolves that it perpetuates the problem it was created to solve. Trivially, those agencies which reliably have achieved their goal get disbanded as no more needed, the only remaining in the long term are such whose goals are elusive.
This is how well-intentioned, honest people try to grab more power for their agency, because it seems to help achieve that elusive goal.
(To say nothing of people with less integrity in a position of power.)