Why do ants easily squished in your fingers while lizards don't?

Because ants are far smaller than lizards.

Ants being smaller is part of the reason. Another part is that ants are what we call arthropods, animals like insects, Spiders, crabs and so on. Arthropods have their hard and tough parts in thin layers as a sort of tough skeleton on the outside. We call that the exoskeleton, meaning "outside skeleton". Their soft innards are inside the exoskeleton, fastened to special anchors. Lizards are vertebrates; they have their hard parts (their teeth and bones) on the inside, with only the soft, tough skin on the outside. Bones being mostly hard and large, they don't squash easily.

Small arthropods don't need thick exoskeletons, but a small, thin exoskeleton is easy to squash, while a big, thick, bony skull or leg is hard to squash.

Big arthropods like some beetles and lobsters may be quite hard to squash.

But don't squash any animal or plant that you don't have a good reason to squash; they are far more beautiful and far more interesting while they are alive.