Since Most Writers Are Human, as are most of the audience, it's likely for them to make humans something like a standard to compare other races to (These guys are like humans, but short and tough! These guys are like what I wish humans were like. These guys are like humans mixed with animals!). Thus it's pretty easy to imagine them.
As it often happens with the Jack-of-All-Stats, humans are often Skill Gates, but aren't good targets for Minmaxing. Or they may get a few bonuses to charisma or negotiation skills rather than huge perks or flaws. Naturally, this trope ignores that good old Homo Sapiens has quite a few skills we excel at that most creatures cannot match, as well as vice versa. It's also statistically unlikely that we'd fall exactly in the middle in every respect compared to other intelligent species in the universe. Although some games in recent years have decided to add reasons for creating a human over the races that the players view as "cool" or "more interesting".
This can also serve as a reason for making humans the dominant race in a fantasy or sci-fi setting: Being able to readily adapt to any biome or challenge allows us to expand to a degree other, more specialized races cannot match.