Why? Because every single one of these people is designed to fulfil a brief role in the story, they all have a job to do.
For example they can reveal something about your main character -- someone who snaps at a chamber maid tells us a lot about themselves – or maybe their individuality will set a mood, add humour, make things happen; a scowling barman can do a lot for a scene. I use them all the time in my crime fiction.
Warning; if a character who isn’t supposed to matter starts distracting from the main thread of the story, either cut them out or figure out why you, as a writer, are so interested in them.
There’s nothing wrong with a background character attracting attention—as long as you realise that they are not part of the background anymore. The readers will notice them, too, and expect them to play a bigger part in the story.