he/she need to have hands on experience, finding husband/wife, friends or someone they know to pose for them is one way, hiring somebody to pose for them is also one way.
what's wrong with hiring a model to pose? don't models do this for making a living? you question seem like asking why would people hiring somebody with no or little posing experience.
you are generalizing everything again. just like the other thread by you, you still have not clarify what do you mean by commercial shoot, and asking Why most commercial shoot use 2 umbrella?, many members has already pointed it is not truth.
hiring professional models can be very expensive, none professional models are cheaper, wannabe models are even cheaper, friends and someone you know will cost you little or free. So photographers can just pick whichever ways seem fit to him/her.