Because we beleive that cell is structural unit of life and it binds together with other cells
to create a tissue then organ and finally an organism . In biology we often differentiate organisms on the basis of cell number : unicellular ( organism composed of only one cell)
and multicellular ( organism composed of many cells )
hope it helps :) :)
It is because scientists have proved that our body is made up of tissues which are inturn composed of cells. which means, the cells are many in number and are closely knit to form tissues which will form the organelles and organs.