Why do merchants price their items $X.99? Do they think we are idiots?

The $X.99 price tag is common in supermarkets for many items like toilette paper, detergents, and what have you. Now I see the $X.99, even on quality food products. I never liked it. The marketing experts will tell you that when the customers sees $19.99 their brains would register $19 and not $20 and they perceive the price of the item as less expensive. I still don’t like it; I feel offended and treated like an idiot who cannot count. My perception is, instead, the opposite: everything that has a $.99 suffix is not a bargain, it is cheap product.