Corkage is to cover the cost of materials and labour (nice glasses, corkscrew, dishwashing, etc.) and not to cover lost profit from the wine list. Of course some restaurants charge much more for BYO than others (and most don't allow it at all). The premise of your question is a fallacy; many peope BYO and also order beverages from the restaurant; I can't remember ever having BYO and not also drinking something else.