You're kinda using strawman fallacy
Bruisers, by nature, seem to be MODERATELY defensive chamions with MODERATE damage and MODERATE-TO-HIGH mobility.
Juggernauts are HIGHLY defensive and HIGHLY defensive stats and builds, maybe getting 1 core dmg item and 5 defense, but absolutely no mobility
Assassins are HIGHLY defensive, LITTLE defensive, and HIGHLY mobile
Mages are all damage but they make up for it with utility
Bruisers tend to make up almost all skirmishers, but are not exclusively skirmishers
Just because irelia, fiora, jax, camille, vi, warwick all PLAY differently, look at their kits:
Innate defense, Dash, Damage spell, and Ultimate (Some have 2 damage spells with innate defense as passive or stats)
Irelia has Passive(damage) Q(dash+damage), W (defense), E(damage+mobility reset) and minor cc) and Ultimate (a mix of all)
Vi has Passive(Hefty chunk of Defense) Q(dash+damage) W(raw dmg and utility) E(damage) and Ultimate (a mix)
Camille: Passive(Def) Q(dmg) W(dmg) E(mobility) Ult (dmg+minor mobility)
Note the pattern
Now look at most assassins kits
(Damage, Damage, more Damage)
Now look at juggernauts (All damage with INNATE TANK STATS and good baseline numbers w/ mediocre scaling, meaning no need to go hard dmg but better to go def)
Now look at mages (NOT REALLY COMPARABLE TO EITHER OF THESE)
Bruisers tend to be composed of Skirmishers, some split pushers, and frontline damage
Juggernauts are specialists, doing pretty much split pushing and nothing else Nasus is hell to raise as a splitpusher but not THAT good in teamfights because kiting. Yorick is split-pushy too.
Assassins are primarily backline dmg
Make sense?