Below is a radiograph of a high-end adjustable tree saddle.
See all that white on the image? That’s all metal! Granted, some
of the metal is spring steel and designed to flex, but just how much
flex does it have when nailed to a wooden tree? Humans and
horses are fluid, free moving creatures, even with skeletons, we are
designed to bend and flex with movement. So why do we put a
rigid object between two fluid objects? The reasoning behind this
is a support base for the rider. This does very little for the horse
though, who must flex and bend his spine with every step. And we
humans, in order to keep with the horse’s movement need to bend
and flex our bodies as well.
 

When you think about it, does it really seem logical to put a
solid platform between two moving objects that are required to stay
in unison? That would be like putting 4 inches of wood on top of
one of those big exercise balls and then sitting on it and trying to 
balance. If you just sat on the ball, you would feel its shape and
movement under you; with the wood between you it’s almost
impossible to feel the movement until you are face first on the floor!