Well actually they don't all approach at high AoA ! Take the A380 as an example - it has a high wing sweep, but approaches at very low AoA - but this a/c has a very big wing. The other reason why a lot of swept wing a/c approach (and stall) at high AoA's is that they are usaully equipped with leading edge devices (slats, etc). These leading edge devices generate more lift from a given wing area by increasing the stall AoA - and of course the approach AoA with it.