They don't need a full army as they are a peaceful planet with no civil wars going on or anything.
They do however need to protect important people like their queen or senator when going about the galaxy. Hence why they made a capable yet elegant starfighter to escort their transports.
Plus it's not like they kept a large fleet of them. Just a few squadrons at most.
It's just "hence", not "hence why". The "why" is included in the "hence", which means "that's why".
Anyway, it's only the humans on Naboo that didn't have an army, the planet as a whole had a grand one.

