The problem with stock audio is that they are often overused. It can be very confusing for a player when they hear a piece of music in your game which they recognize from media related to a different IP. This can be a real immersion-breaker, because they will immediately wonder "where did I hear this before?" instead of staying concentrated on your game. Background music is supposed to increase immersion, not break it, so this is very counter-productive. They might even assume you stole the track from the other media, even if you just got it from the same source for the same price as they did. So hiring a composer who creates exclusive music can often add a lot of value to a game.
Every development team is different and would have different criteria about who to hire. But some aspects nearly every team would put value on are:
- Understanding the unique needs of game music: Game music must be subtle and not distract from the gameplay. It must (usually) loop flawlessly. It must not become annoying even if listened to for hours. And many more aspects which would be material for a new question.
- Versatility: Games often have very diverse soundtracks with lots of different kinds of music for different situations. A composer who can only produce one genre of music will usually not do.
- Being open to changes: Game music needs to fit the visuals, the narrative and the gameplay. All can change over the course of a project. That means a good composer must be able and willing to change their work in order to better work with the rest of the game. I once worked with a composer who had their "artistic vision" and weren't willing to compromise in any aspect of it. So they insisted that their background track for a menu where the player rarely spent more than 20 seconds must have a 40 second long intro. Needless to say that we went separate ways soon.
- Reliability and Commitment: You don't want your composer to stop delivering the missing tracks when your are almost finished with your game.
- Price: No reason to deny it. Many games are developed on tight budgets and are limited in what they can pay for music.