There's a EU-wide standard for passport photos. Your head has to be properly aligned within a specific frame. You must not show any facial expression. The lighting must eliminate shadows and "shiny" spots.
Regular automatic booths do not check for all those issues. Sure, you can probably create a booth that handles them, but then you will probably also start charging as much as the photographer. And its a one in 5 years affair, so why bother going to a booth?
