Why is the model used to describe the plasma membrane called the fluid mosaic model?

Because proteins and lipid tails are free to move and are not bonded by strong interactions with each other.

Explanation:

The plasma membranes have two layers of phospholipids in which proteins are present. The hydrophobic fatty acid tails of phospholipid layers make the inner core of the membranes. However, the tails are not bonded to each other and make a fluid-like base in which proteins are present. Also, the membrane proteins are not bonded to the phospholipids and are free to change their position within the phospholipid leaflets. This is why the model is described as a fluid mosaic model.