playing on server via minecraft_server.exe, seed is "ccc" (no large biomes)

this is the server.properties file:

generator-settings=
allow-nether=true
level-name=world_ccc
enable-query=false
allow-flight=true
server-port=25565
level-type=DEFAULT
enable-rcon=false
force-gamemode=false
level-seed=
server-ip=
max-build-height=256
spawn-npcs=true
white-list=true
spawn-animals=true
texture-pack=
snooper-enabled=false
hardcore=false
online-mode=false
pvp=false
difficulty=2
gamemode=0
max-players=8
spawn-monsters=true
generate-structures=true
view-distance=7
spawn-protection=0
motd=Private Minecraft Party

We found mob spawners and they spawn mobs, just as intended. But we've never encountered a single mob at night or in any cave! We did not find zombies, skeletons, endermen, spiders, slimes, ocelots, bats. We did find witches, wolves, all friendly mobs and some nether mobs. (we didnt find any blazes, magma cubes or wither thingies yet, but we've just recently entered the nether)