How much of it is "hiring friends" vs just hiring someone you have better information about? A lot of people interview exceptionally well but it's always something of a crapshoot how well they will do in the actual job. But for an internal candidate you should have a lot better information on how well they performed in other roles. You can have actual frank conversations with their managers/coworkers instead of getting BS references from people the candidate themselves chose for you to talk to.