Why don't all cell have the same structure and functions?

If all cells had the same structure and function, it would be like every person was the same person- looks, personality and traits. If all cells were the same, living things like us humans couldn't survive since we have uncountable different types of cells that all have different functions. Every cell has a purpose and a job it does until it dies, and not every cell has the same job. Also, plant and animal cells are very different, be sure to see the differences between them.