I disagree here, about the application of the laws of physics !
The work of a saddle is to distribute weight effectively - I think we all agree on that. However, it's is not a simple problem, like for example, the problem of distributing weight across a flat, rigid surface, such as you might see in an O level physics class. You have to try to distribute a rider's mass, across a constantly moving surface. You have three parts to this - the malleable structures under the rider's bum, and the deformable panels above the horse's back ... and the third part - the interface between them.
Is a rigid or semi-rigid tree the best interface between these different moving surfaces ? How can we be sure ? Even when the tree is flexible - it flexes away from, and back to, one specific surface shape. Which shape should this be ? Think of the horse's back at rest ... and then in motion ... it changes shape all the time when in motion - at which point should we freeze the frame and take our snapshot of his back to use as the model for our tree ? Perhaps his static, 'at rest' shape would work ? But my horse's back, under a rider, in motion, immediately changes shape from his rest position - it comes UP for one thing - and I would hate to build a tree to his DOWN position and hence discourage the UP !
Whatever the answers are - I'm sure tree designers know - one thing's for sure - the tree can only ever 'fit' his back at precisely one point in it's cycle of motion, and will be a misfit at ever other point. Just like a stopped clock that is perfect precisely once a day.
As a physicist, I'd be as likely to dismiss this saddle fit thing by saying 'the laws of physics say so' as to dismiss the three-body problem as 'oh, it's just one more body in the two-body problem' !