The heads are the easiest to resurface because once they're out, they're portable. Blocks are generally ok, but they can get out of true. I have a known good straight-edge that I use to check the block and have not run into a time on a passenger car where the block needed to be resurfaced. Racing applications is a different story.

Cylinder heads warp because they are a small (relatively speaking), long, flat piece of aluminum and they get very hot.

In contrast, the engine block is very large (stronger,stiffer), and it has cooling jackets. Often, they are cast iron (not sure about yours ... you provided zero vehicle information) which is very stable. Therefore the block rarely warps, and the heads are flexible enough to be able to conform to the surface anyway.

Well, it's almost impossible to resurface the block. You'd have to remove the entire engine and find a machine shop that had a block resurfacing machine which very few people have