Well, in the case of the sailor or the criminal, you don't just happen to run into anyone--you're actively seeking people out at a location where they would likely to be. The sailor background also doesn't say you have to actually know the ship or crew, just that you might have served on it or with them. Sure, it says you're calling in a favor, but that also can mean that you're offering a favor for a favor.
Also, while sure, you're quite unlikely to run into someone you actually know when in a different plane (unless planar travel is fairly common in your setting, or the plane is one--like Ravenloft or even the Feywild--where a magical copy of the boat or runner can be made by the powers that be), an instance where the PC is both on another plane and also wants to use their background feature is so vanishingly rare as to be dismissed. I'd go so far as to view that as a bad faith argument, because it's saying "because this feature shouldn't work across the planes, it also shouldn't work if you happen to be in a different place on your own world.
First off, there's a huge difference between "NPCs aren't required to stay at home all the time and sometimes have reasons why they would be traveling; therefore, there is a chance that the PCs and NPC may run across each other" and "here is a small village where everything is made of jewels."
But anyway, isn't that what, like, 75% of D&D adventures are like? The players just happen to be in the location where either some weird event is going on, or someone approaches them, asking them to help deal with some weird event? One of the first Dungeon Magazine adventures I ran involved the players just happening across a tiny village that, in ages past, had petrified a powerful demon in a circle of standing stones.
<grabs Curse of Strahd>
Three of the four hooks are the PCs just happening to be in a location where they meet someone who lures them into Barovia. The fourth hook is a railroad.
<Looks up other adventures>
Descent Into Avernus. Without reading the entire adventure, it looks as though the NPCs just happen to be drafted into protecting the town that happens to get yanked into Hell.
Wild Beyond the Witchlight. One of the hooks is that all of the PCs just happened to have sneaked into a circus without paying when they were a child.
Journeys Through the Radiant Citadel. It looks like the hooks are all either the PCs all decided to go somewhere for a festival, or someone approaches the PCs looking for help, or the PCs happen to know someone who need help.
All of these are quite the coincidence, yes? It's almost like the typical D&D party is always in a place where there's adventure to be had.
So if I were in a party where we came across a town where everything was made of emerald, I'd go "plot hook!" and look to see if the ruler was a friendly scarecrow.