My feeling though was that there is a different between being arrogant, killing a giant spider and just jumping down a random hole that could lead potentially nowhere.
I tend to head down to the Underdark early with drow or deep gnome characters that I have decided are from there. It makes sense that it’s their comfort zone (as uncomfortable as it is) and that they feel more confident of navigating it and finding what they need down there than on the hostile surface. Plus it’s easier to believe that they understand where they’re jumping to.
But I’m with you in finding it a stretch for other characters. I’m not saying it can’t be rationalised, just that it’s trickier. I’ll probably try with my githyanki once I get round to that playthrough, as he’d first go to the Risen Road to find the other gith, so could feasibly come across the Zhent hideout and take the elevator down to the Underdark which seems easier to justify than just leaping into the unknown. Though I do have a couple of chaotic characters who I could see jumping down a random hole just to see what’s down there.
EDIT: For my characters the Netherese teleport runes wouldn’t be any comfort as my head canon is that they only work to jump from one rune to another, and therefore they can only be used when there is a clear path to one rune to enable access to others. So unless my party could be sure that there were more runes down below, they wouldn’t see them as an easy escape route back to the surface. If the game did provide me with an in-universe explanation, as you suggest, and that included making sense of me being able to jump to a rune from the middle of nowhere, then I might revisit this!
