Linklater and Kevin Smith weren't rich kids from what I understand. Though you can make the case they were just lucky. Hell, Smith isn't even all that talented!
They were lucky. And Smith had a good ear for pop culture. They also have a very specific niche. Truthfully, most of the guys who start with even a modicum of talent end up doing commercials because it pays so well. Why spend three years or more trying to get a movie off the ground and completed when you can make a couple of hundred thousand in two weeks? Frame for frame commercials are the most expensive film shot.