Aluminum is a bit of a bastard after its welded or is exposed to any high tempratures.
It can warp and creep over a period of time sometimes years after you have welded it.
Also as well controlling any welding process is critcal to the quality of the weld and the resulting structure.
Its relatively easy to heat treat large structural steel to normalise the stress levels. From memory ALMG alloy you need to get it up to about 350 degs then cool it in a controlled manner 15deg per hour or some such. The bigger the bit being weld the longer its needs to soak at 350 to get everything to the same temp. Also alot of riveted structures have wires and other non metalic substances under them that wouldn't survive getting heated to 350 degrees for 6-10 hours.
Also thin late is hard to get a smooth surface after welding. Just look at boats and what the call hungry horse between the support ribs. The profile of the surfaces needs to be exact on a aircraft.