It would have been unthinkable, in a lot of history, for a nobleman to become a merchant or a craftsman. Not just "people think you're weird," but like your family and friends would disown you and your potential customers would all think you're insane and you'd have no way to learn the job anyway because no commoner would dare take you as an apprentice. The modern Western world has no analog for how deep the noble/commoner divide was.

Third sons wouldn't starve to death in any case, but they would rattle around the family lands making a nuisance of themselves and borrowing money from Dad. Noble families got some income from their tenants' taxes, generally, but it wasn't always enough to maintain yourself in high style. In some times/places, noblemen could get involved in trade, but that would mean funding other people's endeavors, not becoming merchants themselves. And sometimes even that was restricted or frowned upon.