It's not about always being able to stack it quickly, cleaver's synergy with flat armor pen is extremely good and also helps to deal with tanks while dumpstering squishies at the same time.
The way flat armor pen works is it increases your damage by a flat percentage, the less armor the target has the more that flat percentage is.
Taking 30 armor from a 60 armor target gives you a much larger increase than taking 30 armor from a 200 armor target.
Flat pen is calculated after %pen and since cleaver counts as % you can reduce their armor by a lot before the flat pen is counted and massively increase the effectiveness of flat pen.
This is only counting the passive, cleaver has good stats for the price that are pretty attractive to carries.