Copyright law only permits making copies of artistic works when you have license to do so. Youtube only permits use of content it serves in the specific situations described in its terms of service. All other use is prohibited.

You can see the terms of service here:

https://www.youtube.com/static?gl=GB&template=terms

In particular, the first three points in the "permissions and restrictions" section explicitly prohibit tools like youtube-dl. I've pasted these below:

    The following restrictions apply to your use of the Service. You are not allowed to:

    1. access, reproduce, download, distribute, transmit, broadcast, display, sell, license, alter, modify or otherwise use any part of the Service or any Content except: (a) as specifically permitted by the Service;  (b) with prior written permission from YouTube and, if applicable, the respective rights holders; or (c) as permitted by applicable law;
    2. circumvent, disable, fraudulently engage, or otherwise interfere with the Service (or attempt to do any of these things), including security-related features or features that: (a) prevent or restrict the copying or other use of Content; or (b) limit the use of the Service or Content;
    3. access the Service using any automated means (such as robots, botnets or scrapers) except: (a) in the case of public search engines, in accordance with YouTube’s robots.txt file; (b) with YouTube’s prior written permission; or (c) as permitted by applicable law;

As a convenient figleaf, it is also possible to use youtube-dl for some purposes that are not dubious. Of the people I know who use the tool, none of them do that.