I see. I would've thought that if they detected the player they would've tried to find a way to the player by trying to search for a way in (door or hatch leading to the underground) and bash it open if it was within range.
Exactly what they do now. Make a 3 block hole, put a block or hatch above you and they will bash it open. Remember the player is able to place a cement block above himself in a second and dig out of such a hole in 10 seconds. There is no need for a hatch to open. And really, what would you say if zombies could open hatches?
This is true even though its a loooooooong stretch for me. Wood I can see zombies breaking through, and I can even see breaking through cobblestone for the bigger creatures. Cement should be a big problem to break through and steel should be near impossible to break through. But I can live with this knowing that if I want to keep things more realistic I can turn down the block damage that creatures do. I get no option to do that with digging unless block damage changes that too.
Yes naturally, block damage setting works in all directions. This game is special in that everything is destroyable and buildable and the game should and does adapt to its strengths. So since players have super powers of build and destroy, zombies need similar powers as well or they would not be a danger.
This is neither a romero movie nor is it a typical game where all buildings and floors are unpassable barriers. Zombies don't dig in such games, but players neither.
A compelling horde night experience can heavily depend on the player. I'm the type that likes to be out and active during the Blood Moons, though I am hoping they'll add variation in the future like "help defend the trader through the Blood Moon" or something along those lines instead of just the same old same each time. But if a player wants to sit inactive in their underground bunker during the Blood Moon, that should be their choice. Atmosphere wise, there's something unnerving about knowing a truckload of zombies and mutants are up above having a rager of party, and that going up there while that party is going on might spell death in all caps.
This is a horror game, horror games want to keep the player in a state of danger and don't want to give the player the decision to be completely safe whenever HE wants it, at least in vanilla.
But any player can mod the game, turn off zombies any time and turn them on again, turn on god mode, fly 100m above ground, turn down block damage or simply build a steel block 100x100x10m and be completely safe. Lots of ways to be safe and those and many more are enough choices in a horror game where it shouldn't really be possible, if you ask me.
PS: I built underground horde bases in the game as soon as zombies could finally dig, such bases need special knowledge about the way of the 7 days zombie, but they are possible.
It is an intrinsic characteristic of this game that conventional underground bunkers can't work like in reality.