As someone who is fairly ambidextrous, but predominately a lefty - the things that are harder to switch between are some of the gross motor skills.

For example, throwing (or kicking) with your non-preferred side is not as simple as picking up and throwing a ball or simply kicking it. You have to adjust your position and stride to lead with the correct foot. I found learning right-handed pace bowling in cricket (for fun) especially challenging as you have to land your back foot in the right place as you bowl through the popping crease. A few steps and rolling the arm over to spin was easy, and I actually can get more spin on the ball with my right hand.

My theory is that the handedness came about through learning basic survival activities such as running and jumping, throwing spears or rocks, etc that require using a preferred or learned hand.