Manufacturers are quite dependent on retailers for selling their stuff. Even if they would have large operations in internet, the're still would be lots of people who would prefer buying their goods from somewhere else.

Also companies usually prefer specializing to what they're good at and expanding to retailing in large scale could take resources away from something else (since they're always limited). And large retail chains can do this more effectively anyway thanks to economies of scale.

By directly competing with those large retailers these manufacturers could make the retailers quite angry. This could mean low visibility for products, no shelf-space or even retailers not selling these products (I think this happened with PSP Go in some country?).

So from this point-of-view it's best for manufacturers to stick to what they're good at and not taking risks. Having smaller scale web stores probably isn't that big deal. I would imagine that it's more a question of brand building for the companies anyway.

Edit. It seem's I was quite slow with my asnwer.