For a normal eye, objects closer than 25 cm cannot be seen clearly...

The ciliary muscles control the curvature of the eye lens to adjust its focal length. When an object is brought closer to the eye, the ciliary muscles contract to make the lens thicker, which decreases its focal length and allows a sharp image to form on the retina.

However, the ciliary muscles can only contract up to a certain maximum limit. Consequently, the focal length of the eye lens cannot be decreased below a certain minimum value. For a normal eye, this minimum focal length corresponds to an object distance of $25 \text{ cm}$, known as the least distance of distinct vision.

Therefore, objects closer than $25 \text{ cm}$ cannot be seen clearly because the eye is incapable of decreasing its focal length beyond this limit.