Because Smith is quite literally an occupational surname, popular in Europe even before the age of adopted lineage names. Many slaves and immigrants further adopted it simply because it was such a common name, further bolstering the numbers. During both wars, many Schmidts (same meaning) also became Smiths to avoid association with the enemy. Oh wait, you thought you were being clever and didn't really think there was an answer.I dunno, why are there so many Smith's on the planet?