Part of it is that people want to be tricked. If I want to buy something but I'm on the fence about whether it's worth $5 then I think the $4.99 price works as

Part of it is that people want to be tricked. If I want to buy something but I'm on the fence about whether it's worth $5 then I think the $4.99 price works as like a semi-subconscious plausible deniability mechanism to let me allow myself to buy it.