I don’t think it’s related to our design experiences, and I’m also not saying you cannot identify usability issues with unstructured user testing, but if you want to ensure you’re capturing consistent usability issues and not one-offs, you need to use the method properly. It’s not just my opinion based on personal experience, it’s a methodology developed and refined over the years by many usability experts.
Not sure, if you’re already familiar with Jakob’s Nielsen (and his consultancy) work, but I highly recommend going through some of NN/g articles to understand the user testing method better — https://www.nngroup.com/topic/user-testing/. Particularly this article — https://www.nngroup.com/articles/task-scenarios-usability-testing/.
User testing is a VERY important part of iterative design, and I actually say to my students/junior designers — “if there’s only one thing you can do from a user research perspective, make it user testing”, because of its huge value to design. But it’s important that people get it right, otherwise you just de-value this method.