This usually occurs because the game systems themselves are trivial (often a set of independent features stuffed behind the same UI, but A has no influence on B), because one or a few of the systems are overwhelmingly influential (e.g. melee mechanics don’t matter if guns are trivially superior for all scenarios), or because the game itself doesn’t incentivize the player to actually exploit those systems to any real degree (e.g. the game is too easy or the levels don’t introduce enough new constraints, so the first solution found applies globally; often fixed by player-induced artificial challenges like no X runs).
Multiplayer however cheats this because even trivial systems can become a huge exploration space when incentivized by direct competition (analysis and understanding is pushed to the limit; e.g. sm64 requires a certain level of understanding to play effectively, but speed runners and TAS take that understanding to the nth degree) and the players themselves bring metagaming as an additional system worth exploring (eg any competitive multiplayer)
It’s also confounded by multiplayer games that are less “played” and more used as a social space — replacing the shopping mall — like many mmo’s, Minecraft, etc. You’re not replaying so much as visiting and killing time between social interactions (so even highly repetitive actions often suffice).
The content variation is largely necessary to give reason and space to explore the game systems under novel constraints.