So, imagine all the machinery in a real dock.. Cranes, lifts, etc..
If they get trashed, you can't do anything with ships until repaired or replaced.

I guess you could say the same applies in the game.

It's been like that since the game started, as far as I'm aware, however, QoL improvements implemented some fair time ago, your dock now automatically starts repairing after a hit, as do your shipyards. No lost hull build time, & if you were hit overnight, your dock is up & ready to launch fleets.