My theory is many of the women who grew up during the height of the feminist movement in the 60s and 70s are now childless spinsters with 14 cats.
My personal observations fit along with that, too. The pets fill in the void for the “I need something to take care of” niche where children used to be for many women. The invention of doggie purses (replacing baby slings) and pet strollers is proof of this.