That's not true at all. Ewan McGregor has been very successful- he just doesn't chase commercial films a whole lot but that seems by design. Same with Portman (who has an Oscar BTW). And this guy Liam Neeson makes the occasional flick too.
Yes, Hayden Christensen doesn't have much of a career. Not sure if that's unfair or not (I seem to recall liking Shattered Glass and did not like Jumper. Don't know if I've ever seen him in anything else).But the rest of the main cast of the prequels have done quite well for themselves.
Well for one, Liam Neeson was already pretty big before Phantom Menace.
More importantly, I'm talking about major movie careers. To requote myself
I'm talking Ford/Dicaprio/Smith/Cruise tier.
Would you say Ewan Mcgregor is on the same level as Harrison Ford post Star Wars?