Back in the day, They did just that. Lots of airplanes had separate metal gas tanks or rubber bladder style tanks thad just dropped into a hole in the top of the wing.
Fast forward to today and the simplest way to utilize all the available space without adding a bunch of weight is to just build the wing a little more robust where you want it to carry fuel then get out the pro seal gun, seal up the inside of the wing and then stick a gas cap on top.
Yeah, RV tanks are removable, but in a practical sense, it's just another part of the wing, not a huge gaping hole in the wing that has to be designed around. You can still have ribs inside it, don't need the weight or complexity of close out panels on top or whatever.
But yeah, pro seal isn't very fun to work with.