Figuring out that it could be used to write sentences took a few millennia after that. And only after that came enough simplification for people other than palace/temple employees to learn it.
[0] https://en.wikipedia.org/wiki/List_of_languages_by_first_wri...
[1] https://en.wikipedia.org/wiki/Seth-Peribsen
"The original Sumerian writing system derives from a system of clay tokens used to represent commodities. By the end of the 4th millennium BC, this had evolved into a method of keeping accounts, using a round-shaped stylus impressed into soft clay at different angles for recording numbers. This was gradually augmented with pictographic writing by using a sharp stylus to indicate what was being counted. Round-stylus and sharp-stylus writing were gradually replaced around 2700–2500 BC by writing using a wedge-shaped stylus (hence the term cuneiform), at first only for logograms, but developed to include phonetic elements by the 29th century BC. About 2600 BC, cuneiform began to represent syllables of the Sumerian language."
[1] https://en.wikipedia.org/wiki/History_of_ancient_numeral_sys...
[2] https://en.wikipedia.org/wiki/History_of_writing#Proto-writi...
The monumental inventions are:
1. writing
2. paper
3. printing press
4. computers
5. internet
because they are ways to store and disseminate knowledge.
This is sometimes called culture.
Some languages such as Guniyandi have only recently undergone this conversion [0], in fact there are likely thousands of languages still without writetn forms [1]
[0] https://en.wikipedia.org/wiki/Gooniyandi
[1] https://www.ethnologue.com/enterprise-faq/how-many-languages...