They arent blocks just pieces of wood held in the air by another person. You kick or punch to break them. Most things in the real world aren't blocks, that's in your video game, don't get them confused.Lol easily those blocks you karate guys use are simply made to look like you guys are tough boys lol. They are thin planks with air under it thus downward force by a human would render the structural integrity beyond its allowable force thus breaking. But a rotting flesh ridden z would have no chance in all relations to destroying blocks with their dead body parts that only function cause of muscle spasms not strong enough to penetrate anything other then flesh or dirt.
The impact that breaks things doesn't come from your strong flesh. Its from your bones. Zombies having rotting flesh wouldn't hurt their impact power, they also don't care if they break or damage their bones since no pain. The idea that they are stronger than humans comes from them having no limiters like these.
But beyond explaining my perception of what a zombie could be if they existed, this is a video game, about fictional monsters. Who cares what limits you perceive them having. Its doesn't matter at all. And your video game doesn't need to match up with real life anyway, its not a simulator.