Crafting, which includes the creation of foci, enchanted items, and potions is a category of thaumaturgy, thus anyone who doesn't have thaumaturgy (or ritual:crafting, or any thematic ritual) can't make them. If you want to know then why the book mentions trading foci slots for enchanted item slots in the channeler power I'd say that it's because you could have channeler and thaumaturgy (or ritual) and thus have a channeler who could make enchanted items but that doesn't mean that all channelers can.
I'm not sure where I got the idea that anyone with any spellcasting power can create Foci. It seems a fairly strong notion (and a decent reason why channelers would have item slots at all) but I guess I can't find the bit in the book to quote.