I was reading the Wikipedia page on the topic, and it essentially says in the 19th century their emerged texts claiming it, but there is no primary source to confirm it.
So the question is: What, if any, proof is there of it happening. AND why in the 19th century did this begin getting written about?
BONUS: If this is true...WHY THE HELL CAN'T I SALT THOSE DAMN CARTHAGIANS IN ROME: TOTAL WAR II????
It didn't happen. I can't find the article which goes into this, but the amount of salt it would take to actually kill the potential for growth in and around a city the size of Carthage is astronomical. Meaning: Rome would have essentially bankrupted itself for no reason, having already destroyed the Carthaginians. Since it isn't true, you should be more concerned with why RTWII is still such an utter disappointment than why one fun fictional feature isn't in the game.
As for why the theory started being written about, I have only unprovable theories, but I'm guessing it has something to do with Heinrich Schliemann rediscovering Troy, which set off a veritable cottage industry of pseudo-archaeologists searching for "lost" sites.