Who says that the locks in Skyrim are all bought?
Some are found, some are created, some are bought.
One that is created or found could very well be by someone without any real valuables that just had luck/skill.
I agree that not all master locks will protect valuables, but most should.
Realism would involve an increase of the
average value of items for each step up in lock difficulty. There would be few people capable of making the best locks, and anybody that good would be in the trade, selling their work (with rare exceptions). Unless Nirn is a very whimsical (to the point of comedy) place in general, most locks & locked chests would be sold to wealthy people to protect their most valuable property. The majority of this would be cash-valuable, though there would be sentimental items as well.
Over time, chests would fall into the hands of bandits or otherwise spread around the world (a bit), but this is the exception, and it would happen very slowly, because the most valuable stuff would be the most protected--hence the reason for the best locks in the first place.
As depicted by BGS, people in Nirn: leave their most valuable items laying around, store a few less valuable items 5 feet away protected by a cheap lock that a blind, feverish child could open, and store their junk 5 feet further away in the highest quality locks that exist.