You are confusing several things.We do however have many archaelogical artifacts that testify the existance of those populations. My knowledge on the subject mostly comes from recent school teachings, so it may be heavily biased and with many assumptions but I think that they wouldn't teach of populations that never existed. I know there was no Italic culture as a whole, sure, but there were many smaller cultures or populations that were living in Italy and are refered to in Italian as "Italic" which actually is the name of a specific population that lived in Calabria
"Italic" languages do exist. They are a fairly notable ancient and classical language family, the speakers of which at one point were settled throughout most of Italy.
Material culture is different. Not everybody who used items of, say, the Villanovan material culture spoke the same sort of Italic language, kinda like you don't have to have spoken Greek to have used a pot that looked like this. It's not even a very good metric for that sort of thing.
Neither material culture nor the language one spoke correlates all that well to other aspects of culture, like the gods one would've worshiped or the stories one would've told. In southern Italy, something of a mixing bowl of languages in the early classical period but mostly populated by people who spoke Italic languages like the Sabellian languages, you could easily find people who a) were natives of the city of Rome, b) spoke Latin, c) employed chiefly Greek-associated material culture, esp. pottery and statuary, d) worshiped "new import" gods like Isis or Sarapis from places that weren't even in Italy, and e) still had an interest primarily in the myths and tales of the local Sabellians.
And none of that has anything to do with genetics. You can't tell if a man was a Roman senator or an Auernian bondsman by looking at his DNA any more than you can tell if I'm German or American by looking at mine.