I don't think the ants are any tougher than they ever were.
I think the humans are getting dumber and less effectual at everything they do. You see people in third world countries getting their asses kicked by insects all the time and they are unable to organize a counter offensive. Sure, they'll allocate some money to fight the problem, then a huge list of bureaucrats will siphon parts of that money off, each dragging away a little piece until there is nothing left for the problem itself - like ants themselves. Everything in these kinds of countries is called a bold initiative and a real plan of attack. A year later, nothing seems to have happened other than desperate solitary measures and the problem is worse.
Back in the 1950's, America decided to solve the malaria problem in Africa. People who thought they knew what they were doing (they didn't) blocked spraying of DDT, citing studies later shown to be completely contrived. The loss of life since then to malaria can be calculated in the millions. This was back when America was more organized than a clusterf*ck in a madhouse and could carry out a large scale program for any length of time.
In third world countries, you always have people talking about the problem, as if words were a kind of magic chant and intention could solve it without any substance. At every turn, people in third world countries tend to dissolve after meetings into individually struggling factions that are at war with themselves. The ants and insects are superior organizers compared to the humans in these nations.
It's not the ants that are getting dumber. It's the humans.