Only a tiny amount of moisture is needed to maintain corrosion (plus oxygen, of course). Given that the metal surfaces inside the box-sections and double-thickness spot-welded joints are unlikely (in manufacture) to have much paint on them - if any, then corrosion is inevitable from the start.
It doesn't matter if chassis members are full of water or visibly bone-dry, corrosion will carry on due to the high moisture-content of the air to which the chassis is usually exposed - inside and out. A misty day = loads of moisture (as much as air can carry, 100%); a normal summer's day here in UK probably has minimum 10-20% moisture still. Normal house conditions are around 10-15% moisture inside even with central hetaing. One water molecule touches the bare steel - yum yum, iron oxide starts growing.
With older cars, only those kept in desert-conditions and air at about 0% moisture survive rust-free.
Modern cars with galvanising can survive mostly rust-free for many years due to the protection of the Zinc coating that galvanising provides. The Zinc corrodes in preference to the steel (it is more chemically reactive with air/water), until the zinc has all gone from an area. Then the steel starts corroding. 'Modern' is mostly from the 1990's onwards for forward-looking car manufacturers (Volvo, Audi etc), otherwise from later on - most manufacturers now at least Zinc-dip the lower parts of their cars I think. It costs money and time, so I imagine as many car-makers as possible try to minimise the galvanising, but reputations can be broken if corrosion is rampant (remember the Alfa-Romeo Alfasud ?).
Aluminium only corrodes slowly (mostly in salty conditions) and plastic just slowly weakens - these are now used in places where normally was steel, so reducing the rust problem even more..
For the older cars in the world, some galvanising and other dipping or spray-injecting to coat all bare metal with an air- & water-impermeable layer is the solution, as we all realise.
End of lecture!