L. humile isn’t your stereotypical ant, with one queen and many workers laboring in a single nest. Argentine ants have multiple queens per colony, and there can be as many as 300 queens for every 1,000 workers. This makes them virtually impossible to kill with poison bait traps, which work on the principle that workers bring the tasty toxins back to the queen, whose death destroys the colony. When you have a lot of queens, that’s not an effective strategy.
An excellent article by Annalee Newitz on Argentine ants.