There's a rather big difference between doing regular roles and actually being successful, I would think.

Certainly, it takes some luck sometimes to become an actor in a leading long-running show instead of one that's cancelled halfway through the first season. But it's that luck that for many of them seems to have been missing.

As for SMG herself, sure, she's acted in some movies since then, but for the most part those weren't exactly a success. Of course, as a foreigner, my view of American television is a bit selective, as the shows that become famous internationally aren't necessarily entirely the same as the successful shows in the US (HIMYM is a case in point). But even so. It's been eight years since BtVS ended, and when most of the main characters haven't had any long-term roles in major shows since then, I think it's fair to say they haven't had much success, as a group.