They can't leave as early as the men, there are restrictions. For many, it comes down to whether they play out their eligibility when they have sat out a season in the middle of their career (injury / red shirt / transfer). There is an age restriction whereby a slightly older player might be able to leave earlier than a younger one, as well. Some that meet the requirements have skipped their senior seasons, but it is actually pretty uncommon. I would say that most with remaining eligibility (certainly not all, Cappie stayed) tend not to stay post graduation (of course, a lot do also transfer that don't have WNBA ahead as they can play that year immediately under transfer rules post-grad).
Piph left early, skipping her senior season, but went overseas as a pro because she was not eligible for the WNBA draft having not met the requirements. This is even more uncommon, although she is not the only one I believe.