[Suicide Junkie]

Mainly because stock generally sucks.

You seem to be thinking of the ship classes in SW as hull types. That's going about it way too iron-fisty.

An X-wing is simply a large fighter design. The X-wing class ships have 4 guns, a shield and a hyperdrive because the rebels designed it that way.

The Tie Fighter is a small fighter much like the rebel headhunter... they use different shipsets, and designed their small fighters differently.

See the perspective?