Why do ants die immediately upon trying to enter my house?

Not sure where to ask this. Every summer right outside the door to my house, I get a big pile of tiny dead ants lined alone the bottom of it. We never get any ants inside. I live in a house that's over 100 years old so it's not they just can't get in, it's like the house is killing them when they try to enter. I looked it up and apparently it's possible the house was treated with chemicals that prevent bugs from coming in, but the practice seems to have stopped before my house was built.

Any ideas?