That's a good question. I believe the answer is that their weight isn't enough to break the surface tension of the water. That's an amazing photo of the ants' natural instinct of survival............
Photo of one of my favorite bugs.........Water Strider
Yep.
Get some dishwashing soap and squirt it on the water near them and see what happens.
Just don’t squirt it between you and them.
They’re nasty little beasties.