Putting your work into a portfolio is usually a standard because it allows for there to be one website where all my information and work are accessible. I can have my sound and music all on one page to allow for easy access, plus my contact info and resume are also already on the site, negating the need to search for it in the description of a video or bio of a youtube channel. It's the same reason artists don't put their paintings up on Imgur when they are shared. It also looks more professional to have your own website and not risk having another sound designer's work be recommended in the suggested videos. Lastly, youtube has an awful copyright system which means that any of my work could be taken down for perceived copyright infringement, even if it's my own work. Long story short, My portfolio is going to have everything on it that a youtube channel will have, plus more