The deformations in spacetime are permanent (hypothetically). The mirrors in LIGO, however, are subject to all the other various things in the environment tugging on them. They return to a state of alignment because of their environment (local gravitational forces, etc.) This "noise" would prevent a detector like LIGO from seeing the permanent changes to spacetime.

https://www.quantamagazine.org/gravitational-waves-should-pe...

https://news.ycombinator.com/item?id=29485803