Why does every RPG have orcs? It’s important we never start doing things out of convention, we should seek to maintain the knowledge of why these things were implemented in the first place.
D&D essentially conjured the pig-faced orc as an amalgam of sub-human henchmen picked from across appendix N. Tolkien’s orcs perhaps cast the mold but like most things from early D&D it became its own object. Their pig-faced nature apparently being credited to a miscommunication between Gygax and the artist that illustrated OD&D in 74 and Swords and Spells in 76.
AD&D is a kitchen sink game. The reason for this, like much of the wisdom it’s pages hold, is implicit and only becomes clear 5 years after you start playing it. From a mechanical perspective orcs are fairly redundant with many other low hit die mobs. Bandits, goblins, hobgoblins and so on. So why bother?
When writing a novel or a short story you have a captive audience. The reader will read all the words on the page. They can parse and track a fair few different groups and individuals before it becomes too much and they need a dramatis personae or some such. TTRPGs are technology which produce narratives but playing one is not similar to reading a narrative. Asking players to track that the Grisnahk tribe is making war with the Shrevgak tribe is much more a pain in the ass then telling them this orc tribe is warring with this hobgoblin tribe. The thematic space these two monsters occupy may be so similar that in the context of writing a story it seems entirely redundant to include both but within a game its a matter of practical utility.
Monsters have connotations much like words. Choosing one over the other is a type of short hand communication. Lizardmen in the temple? This place must be antediluvian. Bandits on the roads? Perhaps the local lord is losing his control. They are serving different roles but their connotation spins the scenario differently. Players being able to parse clues for what’s going on beyond their line of sight is a huge part of the strategy of navigating campaigns. Furthermore, they simply provide variety.
RPGs are big games. Even ones with compact rulebooks represent months or years of drafting and consideration. They are also intended to be played for huge spans of time. A campaign which lasts even 6 months of real time engenders at least 20 sessions. If each session runs a respectable four hours that’s 80 hours of gameplay not including preparation and planning on the front end. Good campaigns run (much) longer than this, and a system should be fresh for more than one campaign. Hundreds of hours of elf games. This is the most important way in which RPGs and books are different. Monster variety is future proofing the longevity of the game. In one campaign, evil men may be your primary foe and in the next its gnolls. It is a thematic and aesthetic concession for the health of the game itself, so that a new edition of the game doesn’t have to be written for each campaign you run. RPGs are full of these sort of compromises.