A crutch for what? A shortcut for what?
I don't think that RPing Legolas is an offensive stereotype, lol. While I don't like racism even against fictional races (because it's a sign of something worse in that person), I don't think RPing a stock character is the same. Are there not human archetypes that people play into- aka, the boring fighter, a generic action hero, the edgy rogue, and so on? There's no problem that's limited to alternate Races and no merit that's limited to Humans. And even if it is shallow and exaggerated, it IS role playing. You can't say that it isn't.
I don't have to show you a table that's mastered complex Humans because it doesn't matter. You don't have to graduate the Human tier to play another species- I apologize if I am misinterpreting you, but it seems like you're acting on some sort of rule that someone has to put fleshed Human RP first. "Added challenge" smh. This vibe I'm getting is why I'm trying to say that Humans are just another fantasy hat, and are not more meaningful to play. My argument is that any character that can be played is going to be limited in depth, so playing characters with huge cultural variations is one of the few ways to cut through the mire of laziness with a new perspective. You get something different out of playing an Elf than a Human. The depth of your character does not depend on the race, it's the player. If I showed you a table of borderline Shakespearian Tortles, would you be satisfied?
I can concede humans aren't inherently boring, but "they haven't given much thought to the complexity of our existence" is WAY out of left field. For all intents and purposes, the other races are still people with people problems. Thoughts on life, death, what it means to "be," etc. are universal themes of sapient creatures. You don't have to play a Human race to explore real-world existential perspectives.
My original response to you was because you made the following assertion:
"Playing a human is 'normal' because we already know what those shoes are like to walk in, so it's not like you're really broadening your horizons or imagination."
That is, imo, incorrect at best, risible at worst. Humanity is complex and difficult to understand. In fact, I would go even farther; it is all we have to understand in terms of complex intelligent life. All of the 'extreme' RP models we are using- they have a basis in humanity.
So for someone to say that they cannot play a complex human, that they are unable to broaden their horizons or show some imagination when roleplaying a human? Well, that's, you know, your opinion, man.
I am reminded of a conversation I overheard at a restaurant. One person, who was enjoying the excellent sashimi, was complaining about some French food he had the other day, and said (without irony), "What have the French ever done for cuisine, anyway?" ...ahem...
There are perfectly valid reasons to play non-humans. But to say that they offer a roleplaying experience that is not offered by humans is not, in fact, correct. There is an inherent tension in the last paragraph where you are waxing eloquent about the thoughts of life and death of all sapient creatures; wait, you mean humans, right, because humanity is the only sapient creature we are aware of (or, at least, that has a complex system of communication for these ideas- I don't want to slight any pro-dolphin people out there).
Which means that humanoids, demi-humans, aliens, and other IMAGINARY sapient creatures will always be caricatures. Sometimes they can be valuable explorations! Sometimes they will provide people with "rails" and "rules" for their roleplaying (be dour, drink, and have a scottish accent), but these are crutches at best, and caricatures at worst.
It can be fun, but the purpose (if you are into roleplaying) is to explore humanity by emphasizing certain traits. Not to assert that you already know everything there is to know about humanity, and therefore have to make up new sapient critters.