The reason why hydrophobic molecules can easily cross the cell membrane while hydrophilic molecules cannot lie in the composition and structure of the cell membrane.
The cell membrane is primarily made up of phospholipids, a special type of lipid that has a negatively charged, hydrophilic phosphate head and two nonpolar, hydrophobic tails. The phospholipids are arranged in a way that the hydrophilic (water-loving) heads are facing outward and the hydrophobic (water-hating) tails are facing inward, forming the characteristic lipid bilayer.
The hydrophobic inner layer allows hydrophobic molecules to cross the plasma membrane easily because they are both similar in their hydrophobicity. However, the hydrophobic layer does not repel hydrophilic molecules and does not allow them to cross the cell membrane without help.