Why does foam dull knives?

"Soft and squishy" is a bulk property. That's not due to either the polyurethane or the air fraction individually, but of the combination.

However, at the scale of a knife's edge, that foam is in fact locally either air or polyurethane. Cutting the air bubbles is trivially easy, but that small fraction of polyurethane is actually quite tough. So, while you're putting only a few newtons of force on the knife, at the edge those few newton are concentrated on only a few points. Let's assume that each point where you're cutting the foam is 0.1mm square and the force locally is 0.1 N. That gives a pressure on the edge of 10 MN/m2, or 10.000 bar. Quite a lot.

My guess is that the foam has some type of sillica in it which may stick to the knife having a detremental effect on its cutting ability possibly cleaning the blades regularly with alcohol could help this but i'm no chemist, hopefully this is helpful

Having previously done academic research on various aspects of knives, I agree with @MSAlters - the actual polymer you're cutting is quite tough. The practical solution is to do what professional knife-users, like butchers, carpet layers, whatever, do: run the blade through a hand-held sharpener after every few strokes. I now do this when indulging hobbies, whether cooking or carpentry. Works!