Money. It's cheaper to do it that way... touchpads are generally PS/2, keyboards may or may not be USB (probably also PS/2, actually) -- but the doozy with them is that the controller chip is on the mainboard to eliminate yet another PCB that isn't really necessary anyways. I believe cameras generally ARE USB, but not with any connector you'd find familiar. Sound is generally integrated into the chipset, and emulates a PCI or USB device. Sometimes there are external codec chips... it's not common except on systems with chipsets so awful that sound just isn't included. Thin clients, for example.
There's also the form factor to consider -- to an extent, there are no standards for laptop gadgetry because there's no way to do that. Different laptop sizes and shapes require differently-sized and -shaped crap inside.