The story of why mammals keep evolving into ant eaters is kind of like the mammal-evolution equivalent of how almost every Reddit fan page evolves into an anti-fan page There are a lot ants out there, so a bunch of mammals eat ants, and so a lot specialize in eating almost only ants. Attention flows to negative commentary, so a bunch of online commenters post negative stuff, and eventually a lot of them specialize in posting only negative stuff. Over a long enough time period, everybody becomes what they consume.