You're conflating "cheap" as price with value ((quality & quantity)/price). Junk is still too expensive if costs too much. Jack in the Box is terrible food but the location just south of the Googleplex on Shoreline has outrageously high prices ($20-25 average with tax for a male late teenager or large adult male). That's low value crap. If it were monetarily-cheap, in addition to nutritionally-cheap, then it might make sense every now and then.

They manage cheap by local standards in many places, but they usually have to use more expensive labor and ingredients in Europe to match both legal requirements and customer tastes.