their scalings might be very high, but the way these champions are playing doesnt allow to abuse those scalings for full damage builds.
even if you build crit garen, building him glass cannon with 6 damage items is a terrible idea and doesnt play out well. the same goes for ialloi, and in most cases amumu and other fighters or tanks.
you cant just look at health and defense scalings alone. for example garens villain mechanic makes him massively scale better with defense and cdr than with damage. as long as you can manage to get to the villain and keep him in range, youll always deal a respectable amount of damage. so its usually more worthwile to focus a lot on defense than offense.
a damage build gives you a bit more freedom to chose your target, but is also more difficult to pull off, as you blow up faster. you rely a lot on the momentum of catching your enemy by surprise. just building glass cannon and charging straight into a tamfight gets you killed before you get into range of someone, even with the % damage reduction from courage.
illaoi has a similar problem. you cant really build her glass cannon, as adc would just kill her from outside her effective range even faster. she is kinda like heimer, in that she needs to build a nest of tentacles, where she wants to fight you. leaving the area where tentacles are around means she has absolutely 0 duelling potential. thats even worse on a glass cannon build, than on a tanky build.
amumu wants to be in the middle of the enemy team as long as possible. he can deal 5% of multiple peoples max health per second in damage over 20 seconds, or 8%/s over 3 seconds before he dies on a glass cannon build.
in those cases it comes down to. can you get your rotation off more than 2 times with more risk involved before you die with a glass cannon build to be more effective than on a tanky build? or can you get your rotation off 4 times with a tanky build while dealing slightly less damage, but still enough to kill someone? overall the defensive build will be more efficient.
also, additionally specifically in case of nautilus and illaoi they also have health scalings in their kits (and technically garen with his passive and his w), that further increases their synergy with defensive items.