Mobs digging through the ground has the potential to wreck havoc on players. For example, what if you're in your house smelting some iron and all of a sudden a land shark jumps up through your floor and kills you? Or you've finally finished that scale replica of the Eiffel Tower, only for a swarm of Oozers to start eating it up?
The underground would be completely ruined with little one block tall tunnels everywhere. Imagine you're mining and you for the first time diamond ore- and then a Dowser shows up and swallows it whole!