One reason why they have level 1 is because it is a game. If characters started at level 5, this would mean they could reach much fewer levels in the game.
Character development is important in RPGs and unfortunately 5e is a huge obstacle in this.
Secondly, if the character started at level 5, the player would have to make virtually all the important choices in character development when creating the character.
This is definitely not a good thing for people who haven't contact with 5e.
After all, level is just a number anyway, and it definitely shouldn't dictate history.
I think the truth is somewhere in the middle. Level shouldn’t be a strict indicator of what kind of character we are dealing with. Though we can all agree that there’re some limits to this as well. Khalid & Jaheira is a good example. They’re low level because of balancing purposes obviously. On the other hand they’re not Harpers leaders, or any kind of legendary (yet) heroes, so here’s less dissonance overall.
On the other hand I don’t see any problem in meeting high level companions later in game. Quite the opposite — I think trying to make best of pregen characters is an interesting minigame. I don’t really like minmaxing in pursuit of achieving a best optimized party. When playing IceWind Dale I limited myself in creating characters with help of pretty sophisticated mechanic which didn’t allow me to simply dump points in a desirable stats.
