Why does a very small aperture not give more sharpness?

Because beyond a point diffraction smears fine detail. More depth of field then comes at the cost of overall sharpness.

Diffraction Blur

Category: Capture

Loss of sharpness from light diffraction at a very small aperture. Limits how much detail a lens can deliver regardless of sensor resolution.

Diffraction blur occurs when light is diffracted at the small opening of a strongly stopped-down aperture and spreads slightly on the sensor. As the f-number rises, depth of field increases, but beyond a certain point overall sharpness falls again because diffraction smears fine detail.

This explains why more sensor megapixels do not automatically mean more detail: with medium format the optimum is reached around f/8. Gigapixel photography therefore uses a low-diffraction aperture per frame and gains depth of field through stitching and focus stacking.