u/Toucan_Lips put it best with his comment

“You get a lot more surface area cooked very quickly so you get a lot of crusty edge. Also usually one portion is split into two patties so you get two thinner, crustier patties that are then held together by melted cheese.

Compare that to a more classic burger patty which is often thicker and has a juicy/pink interior and an outer crust. Smash is almost all crust.”

Hopefully you can find a mom and pop shop that sells them. They really hit the spot.