from what i know

during the flight as fuel is used up, the wings become lighter as the fuel inside them is used up. perhaps wings that are lighter are better on landing? not sure…

i think its advantageous if weight distribution can be altered across the aircraft, for whatever reason. fuel can be transfered across the aircraft from tank to tank to improve performance/weight distribution/flight load relief/trim change/in flight safety etc during flight – although transfer possiblities are limited to the crew. weight in the form of fuel is taken from the wing and put elsewhere.

if there was baggage and cargo in the wing, and changes to performance/weight distribution/flight load relief/trim change/in flight safety needed to be made by moving weight from the wings, it wouldn’t be possible – you can’t move baggage and cargo in flight.

the wing is designed to bend as lift is created, but if the wing is filled or partly filled with solid items like baggage and cargo, it might affect how flexible the wing is – the fuel, being a liquid allows the wing to bend without so much restriction.

also, for lots of types of cargo, you need a pressurised space. whilst fuel is also pressurised, the pressurisation for the fuel isn’t the same for cargo. the fuel is stored at very cold temperatures, and the contents of the wing gets very cold – if cargo was stored in the wing, it would have to be heated to stop your clothes in your baggage from freezing. you would need space wasting heating systems and pressurisation systems in the wing to store baggage and cargo.

also the shape and structure of the wing probably isn’t suitable for baggage and cargo. there really isn’t much space in a wing for anyhting other than fuel – being a liquid it can get into lots of spaces – you can’t shove a solid suitcase into a tight space. there are lots of ribs and stringers forming the wing, taking up lots of the large, open spaces, so you end up with lots of smaller spaces which i should think could only be filled efficiently with fuel.

access also probably isn’t the best. how would the cargo be put into the wing? from the bottom or top? new equipment would have to be designed.

you mention engines being put on the fuselage. if the engines remain on the wing, and fuel is put elsewhere on the aircraft, i.e. not the wing, more complex fuel feed systems would be needed to take the fuel further across the aircraft to the engines. with much of the fuel on the wing, next to the engines, the fuel has to travel shorter distances so a less complex fuel feed and fuel transfer system is needed.

safety is another reason. if all the fuel is located in the fuselage, there is perhaps a greater fire hazard and danger to the passengers as the fuel is closer.

i write too much…
if im wrong on any of that, someone feel free to correct me.

thanks

I agree with all your points. Especially the pressurization didn’t come to my mind.
The best would be if the wing always keeps its weight. In some situations a loaded wing is better than an unloaden. Problems occur especially for flutter and gust loads.