I totally disagree.
It's true you will probably learn nothing from video, but the idea is to follow along. All the classes I ever followed on Udemy were about building a practical application, which you can do by following along with the code (+explanations) on video OR (as some advise) pick up just enough base knowledge and continue yourself and use the videos to check up.
But the underlying point is that different people learn in different ways. I would never be able to learn by watching a video, but I do follow along in Udemy courses and write the code myself, run it and sometimes do additional research (as no course is 100% perfect).
Finding your own app and learning 100% on your own can work (if you're that kind of person) but it wouldn't work for me. Courses are ok because they come with the practical application to build (so I don't search for the idea - it's not a bad thing per se but if I want to learn Python, I don't want to waste - as you say - days just looking for an application idea that resonates with me and makes me feel it's worth building) and put the focus of the actual work to do on the thing you want to learn (language / framework / etc) alongside some decent "best practices" (varies from course to course, from author to author) whereas just building myself I might very well pick up bad/outdated practices from whatever random articles I find online.