Why can't a single celled organism have complex systems?

Single celled organisms or prokaryotes lack complex systems, which are a necessity to multicellular organisms or eukaryotes. Prokaryotes carry out all the life processes at very simple and efficient level.

In comparison, eukaryotes have complex systems (such as circulatory, nervous, digestive systems, etc.) for carrying out life processes. Complex systems are composed of several cells. Cells that carry out similar functions, group together as tissues (such as muscle tissues or nerve tissues). Tissues with similar functions are grouped as organs (such as our heat or lungs, etc.) and they carry out a common function (or functions). A group of such organs working in conjunction to achieve common functions are known as organ systems, such as circulatory system.

Thus, we can see that complex systems are composed of a large number of cells. Single celled organisms lack multiple cells and cannot have any complex system.

Hope this helps.