Because you'll have a continuation of the current system of non-stop litigation bankrupting families, continuing to drain tens of billions of dollars of taxpayer money to fund a system for people to litigate over children.
The point of a rebuttable presumption is the flip. You start with the scientific view and the view of the public. Remember what the public wants. You start with that view and say, unless it's disproved, we can be pretty comfortable that it will be okay. It'll be in the children's best interests. That's what the science tells us.
We don't need to worry and do these huge expensive studies. The average family can't afford a two-week trial. The public can't afford all these conflictual families to have two-week trials, so we say we know they're going to be fine. We can't do any damage with equal shared parenting for a normal family.