AbsolutelyNotAVelociraptor

We use it when, for any reason, a person who could easily use something related to their field, doesn’t use it. What it means is that if someone who could be using something because they know how it works, isn’t using it, there must be a reason.

In Spanish, we have a saying: “En casa de herrero, cuchara de palo”.
A rough translation would be “in the blacksmith’s house you’ll find wood spoons”. It’s not a new thing, it’s been like that since ancient times.