Hiring people is easy. Hiring the right people to fit your purpose requires more intensive questioning. There's a lot of psychology in putting together successful teams, and one non-player can wreck everything.
As a hiring manager, I have a standard list of questions I ask, as well as a set of specific questions directly related to the position we're interviewing for (which are generally more technical). Some of them are probing an applicant's thinking and problem-solving skills, and some of them are me trying to figure out if their personality is a good fit for my team.
Part of that is understanding an applicant's motivations and goals. So you get those "what do you want to be when you grow up?" questions, because it helps a hiring manager understand what's motivating you to apply. And I also ask the "give me a great success in your career and a great failure, and what did you learn from each?" question. Anyone who says they've never soiled the sheets working at a previous gig is either lying or has never experienced failure, and they can be a risk to hire.
There's also the "get these people across the bridge in as few steps as possible" kinds of questions that are purely exercises to understand an applicant's thought process and critical thinking skills. Even the most menial fry cook needs to think their way through how to get things done the most efficiently in a rush, so they aren't dragging down their team when it's important. Understanding how someone thinks their way through a problem matters. Exposing them to a little stress in an interview can give you an idea of how will they will handle really being on fire later, so hiring managers will probe to get the info they need.
It's all about balancing skill, problem solving, and personality, no matter how low or high up the chain the gig is. THAT's why you get asked those inane questions.