Why make a bounty hunter ship with no interior?

Traversable interiors in general make a ship considerably larger - something you've had the opportunity to witness in the development histories of some of our other ships. Larger surface area and internal volume can in turn translate to higher mass (see https://robertsspaceindustries.com/comm-link/engineering/16170-The-Shipyard-Ship-Mass for info on how this affects our ship mass derivations), which quickly runs counter the Hawk's design aim of a light fighter with a small landing footprint. Without an interior, the Hawk can run lighter, cheaper, and be more nimble. Instead of supports and reinforcements to shore up against the structural weakness of large empty spaces (relative to the size of the ship), those materials are more uniformly dedicated to keeping components and resource pipes firmly in place, making them more dense. This also benefits to the lone wolf nature of a player who only wants to pilot a single-seater bounty hunter ship.