Because cyclopically huge rotating things look good.
Vestibular and cardiovascular systems need 100..200 m radius permanently.
The angular speed plays role, too. Because ine can't be being overturned several times per minute minute.
Or the ship should be enough fast to need no AG due to short travel time.
Because a small spaceship is for sitcoms, not for epics.
Because they must carry something, and that's expensive, while the nuclear plants are large.
Because actors take too much money.
Because a small ship allows just run from the school, not to feel like a semi-god.
Because they can't repair everything in flight by hands, so the ship must be reliable and need as few people as possible.
Like the airplanes. Where are all those numerous flight engineers and radiooperators?
Low budget.
If it needs a single pilot, it doesn't need a pilot at all. Because it should be enough aautonomous if the pilot dies/when the pilot sleeps. It's not a fighter which is in flight for an hour/
2 km/s of delta-V is a rather hypersonic rifle.