Yeah, but Omegas were simple evolutions of existing abilities. More damage, shorter cooldown, whatever. Zetas are a sea change with a character--Barris' healing crit allies, Vader auto-refreshing DoTs, Qui-Gon giving allies foresight whenever they sneeze.
A zeta-character is someone you build a team around. You need to differentiate between Darth Vader, who can be in any old team, and zeta Vader, who is always the leader of a team that has been tailored around him as the focal point.
I think "Zader" is silly, too. But there's a value to differentiation, and I'm not going to be an old man yelling at clouds because the young'uns decided to call it something else before I did.