I very recently had the opportunity to watch a professional locksmith attempt to open a common door lock with internal workings that had been worn over time and daily use. I learned something interesting. Even the professionals can run into obstacles that prevent their basic tools from doing the job.
This fellow had a basic set of picks that could easily be carried in a pocket, boot, or fold of a cloak.
So the next time a player complains that their thief character should be able to just walk up to any common lock and easily defeat the challenge, remember, it isn't always as easy as it appears.