Given how old these ships are, how many millions of kilometers of wiring there would be even in a Sword, and how infested with Vermin (that really do like to chew on cables for no reason except extreme risk of death - rats are seriously weird) they are, it probably takes a good few thousand working ceaselessly just to keep the thing operational. It's a full time job for 200 people just to keep the Forth Bridge painted, and the surface area on the humid interior of even a sword is going to be much vaster than that, let alone the exterior and fixing minor battle damage (such as a 50m wide crater blown in the superstructure - not worth going back to port over) is going to be an enormous workforce.
Then there's the standard 40k Trope of Insanely High Tech right next to the pre-industrial. It may be that the ship has Thrusters that can bring its entire bulk onto a new course in a matter of minutes, but to move just one into facing the right direction for a manoeuvre takes 500 strong men ten minutes on a treadmill.
It may be that the plasma reactors at the heart of a starship can put out enough power to light modern day earth, but just keeping it supplied with fuel requires 1000 men full time on hand pumps.
Now I'm not saying they were originally designed that way, but as the ships ages, certain pieces of technology fail that no-one knows how to replace and work arounds have to be employed. The manual labour EASILY mounts up.