Take apart an 1100, then take apart a decent O/U. Look closely at the machining, the precision, etc. An auto can be made with much cheaper machining, with a lot greater tolerances, etc. All modern ones are modular, designed from the ground up to be built efficiently in a modern factory. The trigger group generally contains most of the moving parts, and it's built on an assembly line and pinned into the receiver as a unit. Makes it easier to clean, so I'm not complaining, but it's a lot cheaper to make, too.
An O/U is still designed and built much as it was 100 years ago. The cheap ones are just made sloppily, of inferior metal, in countries with the cheapest labor.
(Note that the more expensive pump guns like the Model 12, Model 31, etc. are no longer made. They're really wonderful, and expensive, too. Relics from another era, they're not designed to be mass-produced cheaply, like a modern 870, 500, Nova, etc.)
I like my 1100. It has brought home some birds. But it's easy to see why it's cheaper to make than the O/U sitting next to it, when you take them both apart and look closely.