Human just isn't that good compared to the other races. I don't know what it's like in 5e, and I get that many people don't pick their race purely for its power, but human just isn't that good in most builds. The extra modifiers you might swing in dump stats going human instead of a +2 race are more of a convenience than a real boon and other races get things like Super Darkvision (incredible for Rogues), Fire Resistance (common damage type), Medium Armor Proficiency (also allows shield use!), or a free cantrip (Want to go Thief but still have Truestrike? Want to summon an invisible godhand for shoving enemies? Can come with regular Darkvision too!).