Because your opinion is in opposition to what we see.
For example: If you take a garden hose and lay it on the ground with a trickle of water.....the flow will snake back and forth wider and wider and wider.
No matter how long, it will not create a great crack like the grand canyon.
With layers and layer and layers, laid over long times, the toplayers will be soft and the bottom, well hard as rock.
But the grand canyon? All layers of hard rock cut steep and deep.....as if all the layers were of the same hardness.
And with that last clause, you jump into magical nutty thinking. Why should it require that all the layers were of the same hardness? Oh, right, obviously it doesn't. Duh.
All it requires is that water should be able to erode rocks no matter how hard they are, which it can.
Duh.