I disagree that they are all outsiders.
When we look at the "roles" of fantasy of science-fiction races, often they are drawn out on a scale from one concept to its opposite. For example, in the original Starcraft, you have:
Protoss: super technology
Terran: some technology
Zerg: little technology
For worlds with a lot of races, like Star Trek, you wind up still putting humans at the center, and then make all the other races Human + or Human -, or a combination thereof. For example, Vulcans could be defined as Human + Superlogic, or Human - Emotion.
But no matter what, you usually wind up with races being more exaggerated versions of narrative human roles. These roles, from my own experience, tend to be things like:
Warrior
Outsider
Innocent
Hedonist
Reformed
Basically a Human
And so on.
Looking at the three Starcraft Races, we have:
Protoss: Outsiders. They are physically very different than humans, and their level of technology marks them as different.
Terran: Basically a Human. Terrans provide the human perspective from which to judge the other races.
Zerg: Warriors. Zerg are the ultimate warriors, adapting and evolving to be the best at it.
Here's where I see the D&D races fitting into these tropes:
Dragonborn: Outsider trope. Dragonborn are more dragon than human, and their appearance and abilities support the idea of being very different than a human.
Dwarf: Warrior trope. Dwarves tend to focus on weapons and armor, and are known for being tough cookies.
Elf: Outsider trope. Elves have a perspective that is "greater" than a human's, being ancient and magical. Sometimes they are nature-aligned outsiders, sometimes they are magic-aligned outsiders, sometimes they are spider-aligned outsiders.
Gnome: Outsider trope. Again, gnomes are very different than humans, being small and magical.
Half-Elf: Basically a human trope. Half-Elves fit into human society, and are often used as a marker for how "other" elves can be.
Halfling: Innocent trope. Halflings are childlike, and often play into this.
Half-Orc: Warrior trope. Half-orcs, being related to the warmongering orcs of the Monster Manual, are known to be good at fighting, and have rules to support that.
Human: Basically a human.
Tiefling: Reformed trope. Tieflings are bad guys (fiends) who have reformed and can join the good guys, and their rules support this narrative.
Now of course no two dwarves are the same, and this is not the ONE TRUE WAY to settle the D&D races into categories, but it's one way to look at it. In my view, Elves are Outsiders.
