She's playing the principal role on a sitcom. That's a full time job for much of16d0 the year. Not impossible to do other projects, but perhaps that's enough on her plate right now. She also has a husband and a kid.
You're right though. She would be excellent in something like Ghostbusters. I could almost see her in a 'Louis Tully' type of role.
I really hope that movie isn't a complete writeoff. It's Ghostbusters. If it's not going to be great, don't do it at all.