By its very nature, a chain on a single speed/fixed setup will generally last longer than a chain on a geared bike, not just because they're thicker, but also because on a geared bike once the chain starts to wear too much causing slipping of gears, it just becomes impossible to ride. With just 1 gear, you can get away with tightening the chain a bit more, for quite a long time, accepting that you're also then increasing wear on the spocket and chain ring.
I've seen some incredibly worn school kids BMX bikes which still work fine.