Children are malleable and relying on their testimony encourages parents to manipulate them. Placing the children with the most manipulative parent isn't in their best interest.

There are so, so many cases of children testifying against a parent while living under the care of another, then recanting years later as adults.

I don't have a good answer here because I think often the experts are unqualified and give bad advice, but it's important to acknowledge the depth of the problem