Hostile mobs on the surface are very rare, they are usually only spawn at spawners, you just got really lucky.

 

I do believe that mobs spawn in the mining world normally, but the spawn rate is reduced and that means you tend to see them only around spawners. when I find mobs away from spawners I find around 3 at a time.

 

And the fact there are already loads and loads of mobs spawning (and taking up spawn limits) at the spawners.

 

Only thing I don't understand is that mobs aren't supposed to spawn in spawners if there is no player in a certain radius. If I quote minecraft wiki's page on mob spawners, it says:

"A monster spawner is only active when a player is within a radius of 16 blocks from the spawner. While the block is active, it will spawn mobs within an 8 × 3 × 8 block area (8 wide and 3 high and 8 long), centered on the corner of the spawner block with the lower x and z coordinates (i.e. the northwestern corner). Mobs can spawn anywhere in this range that is suitable, even if the immediate vicinity of the spawner block itself is not suitable. The horizontal distribution is nonlinear, with positions near the spawner more likely than ones farther away."

Basically, if no player is near enough to a spawner no mobs should spawn there so they should spawn on the surface following these rules (still from minecraft wiki):

"Mobs spawn naturally within a 15x15 chunk (240x240 block) area around the player. When there are multiple players, mobs can spawn within this distance of any of them. However, mobs that move farther than 128 blocks from any player will immediately despawn (see Despawning), so the mob spawning area is effectively limited to spheres with a radius of 128 blocks, centered at each player."

 

Of course, theres dozens of people using spawners at one time, im not sure, andrew probably changed it.

 

tl;dr Finding mobs are hard, guise.

My thought on mob spawning in both Main World and Mining World (due to the fact that these are the only two worlds I have access to):

Main World: Hostile mobs and passive mobs completely disabled, good luck finding one.

Mining World: In the scenario no spawners are active, mob spawning is perfectly normal, both on the surface and underground. However, due to the fact that mobs spawn in a 240x240 area around ALL players in the Mining World, this limit is shared between everyone in the Mining World (usually between 20-40). This means, on average, you'd probably see 20 times less mobs than you would on a normal survival singleplayer world.

However, we must not forget the spawners. Mobs that are within a certain distance of their spawner mean that no more may spawn (?). Remember that spawner mobs due count as part of the mob cap, but once a spawned mob is lured away from their original spawner from a certain distance (16 blocks?), the spawner will continue to spawn, until it hits the mob cap. I do believe that the only exception to this is the Cave Spider spawners, I think they spawn infinitely (correct me if I'm wrong). Spawners overall will decrease the amount of mobs you will see on the surface.

Finally, we must take into account the automated /butcher command. This command activates every 15(?) minutes, and kills all the mobs in the entire world, to reset the mob cap.

BTW, not sure about andrew's setups for the cap for mob spawns.