Woz left Apple a long time ago, so did Allen and Filo (Yahoo Co-Founder). It's a bit of survivor bias and interest in the limelight that the more famous names persist.
If they left then that's a sign that they had less interest or was less invested in the company.
You can't have two people trying to dominate. Unless both founders have perfectly matching visions, their joint command will be as bad as design by committee. Someone is bound to leave or become somewhat subordinate to the other. In the end one person will come out as the main visionary guy for a specific company.
