Because unless lives are on the line, who cares, right?
An example relevant to RPGs is the stat block of an NPC designed to work with another NPC or with a PC. Wizards of the Coast uses "it" for any NPC other than one with a proper name, but I've seen other products that, for whatever reason, use a "singular they" in trait text. If a trait in that stat block gives advantage to "them," is it intended to apply to the NPC or to the NPC and the creature or character it is designed to work with? To know which is intended, you really need a distinction between singular and plural.
You can argue that it is possible to carefully word the text of each trait or ability that might be ambiguous to avoid any confusion, but compared to adding a handful of words and a chance for error to each instance, it seems a lot simpler to just respect the difference between singular and plural pronouns.