Let us say that the DnD world is divided so that 20% are human, 20% are some type of elf, 20% are some type of dwarf, 20% are halflings and 20% for the miscellany.
A rough model, but one that shows that these four races are "common"
Humans play a major role in just about every single adventure
Elves play a major role in multiple adventures
Dwarves play a major role in a few adventures
Halflings have never played a major role in an adventure.
And this is odd to a lot of us. Heck, expand past 5e for a moment. We can likely list off major adventures and events in DnD history that have involved Humans, Elves and Dwarves for multiple days. I struggle to even imagine one that heavily involved halflings. They are "very common" and yet they are never involved. Not rarely involved, not often not involved. Never.
No major force attacks them. No lost lore or lost magic ever affects them. They never live too close to something dangerous. It is bizarre.