I bought a bunch of defective keg o-rings that waste CO2 and emptied a keg on to the floor over night (looking at you: More Beer!). For the price of one retail kit, I put together 10 rebuild kits worth of o-rings at McMaster. In that case (5 hours of researching which $0.05 o-ring to buy), the minimum quantity was more a feature than a bug.

I can understand the annoyance for non consumables, like screws.