I think it's a combination of ignorance and denial.
There are a lot of people who carry a pistol as a ballistic talisman. They want to be technically "armed" but not effectively armed. To accept that they might need a bigger pistol is to accept that there are problems in the world that require that much pistol. To accept that there is a problem that can't be solved with a Keltec .32 is to accept a world that is more scary than they want.
There is also an ignorance factor. If all they've ever had is a basic CCW course, they are ignorant and largely uneducated. They may think that that passing a sobriety course of a qualification shows that they are good enough. They many no appreciation of how limited they are by their equipment. In the interests of full disclosure, I carried just a J-Frame and a speed strip back in the 90's. It wasn't until I started to shoot IDPA that I realized there were holster options that let me carry a full-size pistol.