Probably because it's not as simple as it looks. According to a handy copy of A History of the Ancient Near East: ca. 3000-323 BC, by Marc Van de Mieroop,
"The Uruk IV notations seem complicated to us because seven different systems were used, each of which varied the physical shape of the numeral according to what was measured. For example, a sexagesimal system, relying on units with increments of ten and six, was used to account for animals, humans, and dried fish, among other things. A bisexagesimal system, which diverges from the previous one as its units also show increments of two, was used for processed grain products, cheese, and fresh fish. Volumes of grain or surfaces of fields were measured differently. [...] Although the shape of the number signs could differ between systems, the same shapes are found in various systems but sometimes with different values."
Wikipedia, citing Archaic Bookkeeping
Early Writing and Techniques of Economic Administration in the Ancient Near East by Hans J. Nissen, Peter Damerow, and Robert K. Englund (gonna have to get a copy of that one), claims more than a dozen different numeric systems.
Probably because it's not as simple as it looks. According to a handy copy of A History of the Ancient Near East: ca. 3000-323 BC, by Marc Van de Mieroop,
"The Uruk IV notations seem complicated to us because seven different systems were used, each of which varied the physical shape of the numeral according to what was measured. For example, a sexagesimal system, relying on units with increments of ten and six, was used to account for animals, humans, and dried fish, among other things. A bisexagesimal system, which diverges from the previous one as its units also show increments of two, was used for processed grain products, cheese, and fresh fish. Volumes of grain or surfaces of fields were measured differently. [...] Although the shape of the number signs could differ between systems, the same shapes are found in various systems but sometimes with different values."
Wikipedia, citing Archaic Bookkeeping Early Writing and Techniques of Economic Administration in the Ancient Near East by Hans J. Nissen, Peter Damerow, and Robert K. Englund (gonna have to get a copy of that one), claims more than a dozen different numeric systems.
So, yay, proto-cuneiform.