Why is it always referred to as "common sense" when sense is, in fact, not common? ~

I think It's because we are all are expected to have some sense about things that are common in a social setup.

Common sense is still common.
Only these days common people are senseless.

It is sort of a repeated reminder for public to kindly make it common.

The sense formerly known as common.

Good point!

What used to be common sense is now referred to as a threat to our democracy.

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