Maybe a different way to think about it is not that the skill level of the team is low (because people aren't talented), but that at start-ups, you tend to have

Maybe a different way to think about it is not that the skill level of the team is low (because people aren't talented), but that at start-ups, you tend to have smaller teams or a lot more work is distributed across a fewer number of people. So if you are a young engineer, you might not have access to a good mentor at a small start-up. More onus will be on you to develop skills and learn things by researching and ingesting material outside. At larger companies, they can attract skilled talent with bigger salaries and they will most likely employ more people on teams. While you might not necessarily work directly with someone who can mentor you, there is more likely to be someone in adjacent teams or in other departments who you could reach out to and learn from.