Just built to last. 20 years on, and rust is a minimum. They do rust, just not rusting through, and they hide it under very good thick quality paint, not like certain french and italian brands of the period, where everything was done to save weight, and then they quickly disolved.
They used things like GM power steering racks with steel casings, and GM based transmissions (yes they do manual boxes) from the US, they weigh nothing in a big landyacht, but quite a bit in a Saab, in relation to size.
They had side impact protection beams by default years before it was needed, and airbags and ABS way after it was developed. They tend to build cars that survive real world crashes not those in a lab. And things like big glass surfaces add weight. Plus they are bigger than you think, bigger than a Volvo 240 Estate, quite long for a 3 door hatch at nearly 5 metres.