I couldn't see anyone else having a stab at trying to answer this -- I'm no expert, but my understanding of it would be something like this:
Although you might have small dedicated lines of laptops (think Acer Timeline or something), they do actually share components with millions of other laptops and computer devices. i3/i5/i7 compatible motherboards are in everything these days, memory is standard, they use standard (onboard or otherwise) mobile graphics parts from ATI and Nvidia, they all use similar broadcom (or other) wireless interfaces, they all have mass produced cooling solutions, mass produced hard disks, mass produced optical drives, mass produced LCDs and so forth. Yields are high and sold to multiple manufacturers, so by virtue, the hardware comes cheaper. If you get a laptop that is doing something special - like if it has the Macbook air style form factor, or if it has a 10+ hour battery life and amazing graphics, you quickly find that the price goes above and beyond $500 because it is more specialised. OEMs have bulk licensing deals on the OS, so that isn't as burdensome a cost as you might think.
In a console, everything about the architecture is customised. Memory isn't just slapped onto the board, the whole architecture is designed to work optimally with the other 'known' components, the CPU will be a customised variant of an existing product, but it will have redundant instruction sets culled, special ones added, it'll be clocked and optimised for gaming, and perhaps unified with other components in a System On Chip design. They will attempt to eradicate or compensate for all bottlenecks. There will be specific design decisions made to reflect security concerns, for example - the starlet ARM based chip in the Wii handled a lot of security functions, the Matsushita drive design was designed to perform differently from a regular DVD drive and also had special security instructions built in. In essence, I can say this in a much simpler way - consoles require much more specialised and targetted R&D / testing at the hardware level. Then you should factor in any R&D needed on peripherals, SDK / engine licensing costs (although these should be factored into the cost of a dev kit, the manufacturer may choose to eat costs in order to encourage support), the cost of ongoing support and maintenance... it all adds up.
All of this said, a $199-250 new console could probably perform appreciably better for gaming than a similarly priced laptop will come this fall. At least for a short while.