One the one hand, I totally agree with you. Why are these NPC's that you run into willing to say "lead on, I'll just follow you"?
Shadowheart's case, okay, maybe, she does say "it'll be nice to have company for our final hours".
Lae'zel though? Makes no sense for her to follow anyone. Much less some vanilla, level 1 PC. She's an Alpha. A big, bad-a$$ fighter.

It would be nice if there was something... even the NPC's asking "And why should I follow/team-up with you?". Then, have a custom answer based on the background you selected. e.g. Folk Hero - "Because I'm a hero of some reknown and I know this land". Or Criminal - "Because once we get to Baldur's Gate, I've got associates with connections that should make getting rid of this tadpole easy". Outlander: "While I may be a stranger in these lands, I know how to survive. I'll see us safely to a city where we can get help." Etc. Something like that would probably help address your concerns a bit. At least make the character slightly more 3 dimensional.

Then... on the other hand / playing "Devil's advocate".
It's an RPG. The RP being "role playing". So... fill the backstory in yourself! You don't need Larian to hold your hand and spoon-feed you everything, do you?
Let's say you rolled a Wood Elf Cleric. You can imagine that you've been living in the forests for most of your life but you set out to find other Wood elves that had left for the "big city" to take their chances there. You wanted to remind them of the role of the Elven gods, in case they'd strayed. Maybe even bring then back to the woodlands. But, while travelling to the city (BG, Waterdeep, take your pick), you got scooped up in a Nautiloid raid. You've been held in storage on the ship for days when the game opens. Point is... define the backstory for yourself. Figure out a role and then play it! smile

Personally, I don't worry about backstory when I'm playing a Good Hero.

But trying to play an "evil" character... I just can't. :P
Old habits die hard and I just can't bring myself to pick the "evil" option. So I'm going to have to invent a backstory with motivations just to keep me "in character" if I ever want to do an evil playthrough.