Question

why is a normal eye not able to see clearly the object placed closer than 25cm

Answer 1

because the nearest focal length of eye's lens is 25 cm and it had a feature for change its focal length but 25 cm is the nearest point for a human eye to detect the image

Answer 2

Beco'z our eye has a least dictint vision of 25 cm only. It cannot make it focal lenght less than 25 cm. Hence the reason.

"Other thing Depends on God"(this is not included in answer.)

Hope it helps!!!
Best of luck!!!