It might be because of the moisture that attracts them. After looking up your question, I couldn't find much about it other than the fact that there are ants called "concrete ants", but that seems to be a whole different story. I sure couldn't find a good answer for you .
I suspect that they are sizing up a suitable final resting place for Anty Hoffa, leader of the Anteamsters union.
They think it's porridge.