Well, because the two disagree big-time about childhood vaccinations.
While McCarthy and boyfriend Jim Carrey recently led a march in Washington, D.C., to raise awareness about alleged toxins in children's vaccines, Peet and her screenwriter-novelist hubby David Benioff have made sure to keep daughter Francis (nicknamed Frankie) up to date on all the usual doctor-recommended shots.
"I was shocked by the amount of misinformation floating around, particularly in Hollywood," Peet opines about vaccines in the new issue of Cookie magazine, adding, "Frankly, I feel that parents who don't vaccinate their children are parasites."
Peet is so committed to the cause that she will soon appear in a PSA for Every Child by Two, a pro-vaccine organization cofounded by former first lady Rosalynn Carter.
She may sound hard-core, but Peet insists, "I have a lazy, fluffy, actor-y side." And also a side that's a sucker for the latest and trendiest baby gear.
"I'm the target market for the onslaught of baby products," says Peet, who costars later this month alongside David Duchovny and Gillian Anderson in the new X-Files movie. "This sling, that sling, Baby Einstein, the idea that your baby will be smarter if she uses these learning toys at the milestone."
—Additional reporting by Alexis Brunswick