It's not the valves that aren't holding up as well... It's the seals.
Why?
Because brass is softer than steel. It's more pliable- not as abrasive on the seals.
Quick Couples are a different scenario. For the most part, it's the wear of dragging them across the ground that wears them. (This is even more so on the female quick couplings, in which the locking ring on the outside gets bent out of shape. And that's why I don't recommend having them on the end of the hose, but on the wand instead...) Once they're scuffed up, or distorted in shape, they don't seal well any more. And they get scuffed up faster on the end of the hose.
And they cost more to replace than male QC's do.
Stainless Steel QC's do last longer.