Here's why:

* Complexity: Proteins are complex molecules with intricate structures and diverse functions. Building these molecules requires a significant investment of energy.

* Multiple steps: Protein synthesis involves multiple steps, each requiring energy:

* Transcription: Copying the genetic code from DNA to RNA.

* Translation: Converting the RNA code into a protein sequence.

* Folding and modification: Proteins need to fold into their correct three-dimensional shape and may undergo further modifications, requiring additional energy.

* Constant demand: Cells are constantly synthesizing new proteins for growth, repair, and various other functions. This continuous demand puts a heavy strain on the cell's energy resources.

While other processes like active transport, muscle contraction, and nerve impulse transmission also require energy, protein synthesis typically consumes the largest portion of a cell's energy budget.