Because tree cellulose is very durable. Grassy plants tend to grow very quickly, but produce soft, vulnerable fibers that many living things seek as food supply. Tree cellulose is decomposed by a much more limited spectrum of insects and fungi. Cotton paper is not unheard of, but its longevity does not match that of regular tree pulp paper. There is relatively recent research on using other materials for paper production, such as hemp or cotton, but so far tree pulp and recycling remain the most accessible options for such a robust material.
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