Why are cell membranes considered semipermeable?
Because of the hydrophobic ( water hating ) tails of the phospholipids
Cell membrane mostly consists of phospholipids which has hydrophobic tails. This tail does not allow polar molecules to enter or exit the cells. It does not allow glucose, proteins, etc to leave the cell where as prevents unwanted polar molecules to enter the cell.
Also cell membrane consists of cholesterols whose tails are also hydrophobic and restricts water loving molecules .
Due to this, the cell membranes are permeable.
Note: Water molecules can pass through these tails even though they are polar in nature because they are very small.