Why is it that, the kids always have to be told it was a mutual decision by both parents to part ways?

Why not just tell them the truth?

Mom doesn't want to be together or isn't in love with dad anymore and that's why there's a break up.

It's clearly not a mutual decision. One accepts it that the other doesn't want to be in the relationship anymore and has nothing to do with a mutual decision. One is forced to end the relationship unwillingly.