Not only do you have to figure out the URLs from the source, but you have to assemble all the parts. Youtube's website changes all the time, too, so you'll hav

Not only do you have to figure out the URLs from the source, but you have to assemble all the parts. Youtube's website changes all the time, too, so you'll have to constantly re-read the source and adapt your downloader to it (essentially what youtube-dl does). It may not be rocket science, but it's not trivial either, and not every developer on HN is a web developer or has any interest (or time) to become one.