It's less a convention and more an aspect of a company protecting their most valuable IP's. Granted, as much as I love the FF and Reed, he's not exactly Clark Kent or Peter Parker, but if Marvel/Disney get's the FF going again across media, almost everyone involved will reflexively want to protect him given his historical importance. Reed get's labeled as "boring white guy" but I think this is unfair. The guy is basically Marvel's modern equivalent of Newton and Da Vinci. He's only boring to me when his characterization is reduced to a caricature.
The problem isn't these 60's characters that never age or die, the real problem is that modern creators have no real incentive to create new, potentially iconic, characters. Without question it's hard to create something that sticks with the fans, regardless of era, but I guarantee you that you'd see more attempts if creators were given partial ownership by Marvel and DC.