They might. If they're generated like a PC is and assigned maxed-out Charisma they only have to be level 4 to get Cha of 20.
More to the point, you don't need a stat of 20 to be a believable character.
To be a charismatic chieftain, all you need is better than average Charisma. Assign a stat higher than 11 there and hey, the character is charismatic.
Given that the standard NPC stat bell curve is rather flatter than even the PC one, just sticking a 16 in Charisma won't just make you the most charismatic orc in several tribes, you'll probably be the most charismatic person in the region, including a couple of human cities.
No, but after after learning a little more and more real-life experience, they can.
With the exception of those games where the DM allows the optional variant human rules, even humans can't get a stat of 20 at level 1.
Is it a real-life racist assumption that a human can't start out with a stat of 20? Really?
The reallife racist assumptions are the connection formed by the descriptive language used for specific D&D species mirroring racist language used to describe realife ethnicities. That language will be removed, at which point there should be no links between D&D species and realife ethnicities.
Unless a culture doesn't have boats or the equivalent method of transportation across distances like caravans, that the Ship's Passage feature could be applied to, it does. The exceptions are why I said almost all. At which point, maybe just don't pick the sailor background if the Ship's Passage feature is not appropriate for your character?
Those are likely to be few and far between. At which point homebrewing a feature may be appropriate.
I'm coming up with a blank though: Could you give some examples?
Backgrounds are already eminently customisable. A character concept that you just cannot apply one of the existing class features to would be pretty rare I believe.