Mainly because of the "square-cube law". A cell that is twice as large has 4 times the surface area, but 8 times the volume. This means that the larger the cell gets, the larger its volume relative to its surface area. Since everything the cell gets from its environment must come through its surface and propagate into its interior in order to be used, this sets a limit on the practical size of a cell.
That's one reason why life on Earth instead became multicellular, and evolved organs to move materials more efficiently through digestion and respiration processes. You need the greater efficiency for larger organisms to exist.


