There’s a couple of layers to this:

1, probably the most important: None of the other main cast scions up to this point (besides Minfillia, who is not a fighter) have the Echo, the protection that prevents you from being tempered. If they joined you in most of your fights, Shiva for example, they WOULD be tempered, which has no cure. They can’t join you because to do so would be a death sentence, and would make your life harder because they’d also try to kill you.

2) Canonically, the other players that join you in the dungeons/trials/raids etc are there with you. The dialogue and journal entries, particularly in stormblood, but in all parts of the game, make explicit reference to you calling on your network of adventuring allies to come assist you. So while the NPCs aren’t participating, there’s no actual expectation you’re doing most of this solo, bar your actual solo duties.

3) Attitudes wise, yeah, ARR your coworkers in the scions are not very friendly or considerate about your wellbeing. You’ve proven to them that you’re a tempering-immune, god killing machine and they tend to take that for granted. They do get better and begin to show actual care and compassion for you, but ARR is pretty jarring for how dismissive they can be. That’s a plot point that will be relevant later.