The sedimentary strata folding is the result of the horizontal compression of the recently deposited layers. All the strata that was deposited was still wet and pliable and had not been dried and hardened yet. It is easier to account for all the erosion and folding with a muddy strata than a rock solid strata.
Let’s take the Monument valley case.
Why would the area erode flat? It is because the only part that got eroded was the wet deposited layers. The floor underneath was the existing rock which was solid, so it experienced less erosion than the recent wet layers deposited on top of it. The existing solid rock was like a bathtub floor and the sediments were deposited on top of this bathtub floor. The plug was pulled (at the Grand Canyon location) and the wet strata eroded away until it reached the bathtub floor. According to SG, the entire strata was solid rock. Why would it erode so the valley floor becomes flat and what mechanism could do that?
We can also look at the case of folding.
In the above image, obviously all the layers were first deposited flat. Then folding occurred. If it was all solid rock, why could such a pattern be produced? The only way is if the rock was pliable. In the FM, the strata would not have solidified yet when the compression occurred to fold the layers. In SG, the layers would’ve all been solid rock, so how could such folding occur?