Why is “Smith” a common last name?

The name is occupational and referred to a common occupation, probably more than any other. Other names referred to places or people, but smiths were everywhere, making it a common name. Also, it seemed to become more common because of its commonness, because it afforded some anonymity.

Even in Arabic the equivalent Haddad (حداد=blacksmith) is a very common surname across most Arab countries because it's related to a long-standing occupation which is as old as humanity itself.

Folklore would have you believe its all about chastity belts in the middle ages. The men folk went off to war 'locking up' their woman. The men failed to return and the women needed to remove the chastity belt. So off to the Smith 3 villages away. His price for removal often caused a child to be born who was naturally named after its father. Smith...

becasue there were so many KINDS of smith. one kind for each metal.
and as a mideeval craft name, you had litte control over how you were addresses. Smithing was one of the most common jobs out there
and you here it in many languages Like Schmidt..

Alabama God Smith married Tennessee Goddess & became Smiths

The Smiths really like to bone alot

Someone was Jonesing for it haha

Common English trade name. As others have meantioned there were many kinds of Smiths so that increased the name.

Johnson is the second most common last name meaning son of John. John being an extremely common name due to Christianity. John is Hewbrew for Yohanan which was both the name of an Apostle and John the Baptist.

I would think Singh is the most common...