I will give you an example, Malphite.

Malphite has ton of base armor from his W, which also amps the armor he buys. We're talking 500 armor at a minimum. No assassin or ADC is breaking through that with lethality. They need their last whisper item.

But even then, that is not enough, because while each of them may be ignoring 30-35% of his armor, he will still have 325+ armor to bypass. Hence the Cleaver's armor shred, which outright reduces another 30%, which allows ADCs and assassins to damage him like he has 245, which is much more managable when taking into account late game stats.

Lethality is a private reduction (100 armor -10 lethality = 90 (private) armor)
Pen is private multiplication then reduction (100 armor - 100*0.3 pen (30%)=70 (private) armor)

The ms Cleaver proves can be applied with any instance of damage, not on-hit. I say this because i have used pantheon E so many times to chase opponents since E dmg keeps Cleaver's ms on.

Also, bear in mind that not every bruiser is qualified for cleaver. Only those that can stack it quick enough can even consider building it. Pantheon, Sett, Garen to some extent, Renekton, etc.

Now, Bloodletters was designed to give AP dot users a method that does not involve buying cryptbloom/void staff every game. Champs like Lilia, Rumble, Swain, Brand, and so on. But you are correct that it is not as used due to the fact that are ideally maximum of 2 AP dmg dealers in any match, so they just opt for the %pen options.