Why?IME that is because they are unusual so you need a backstory reason why they are among the more typical races in the world--otherwise it makes no sense for them to be there.
If they were part of the status quo, I doubt people would develop strong backstories for them, either. Otherwise, it is more likely just player dependent I would think?
IME people who pick traditional or typical races tend to develop their PCs history and personality as the world interacts with them. If the DM doesn't offer a world primer or the setting isn't published, thePC of common races start of as stereotypes of their race/class/background and grow from there.
Why can't a player of an atypical race PC do the same and make up a character as they go?
Why must the dragonborn, hippoman, or rockface justify their existence, but the human, dwarf and elf don't?
Which goes back to my point that people are more likely to RP a traditional race flat than an exotic one.