24kt is the standard. I hear what you are saying, but if it were pure gold, it would be uncomfortably heavy at the tip of the pen and have durability issues over time.
As I understand, the gold is more of a show thing. It does have some vibration-cancelling properties, but mostly it is about how nice it looks. Lamy, has been making some super-nice stainless-steel nibs for a while. Like most nibs, they are, iridium tipped. But they flex just fine and write the perfectly.