Comment by origin_path

It's interesting to speculate about why they don't use this much more powerful technology to stop ytdl but instead use this much weaker yt specific thing.

Most likely the reason is that they keep the botguard system for the stuff that matters to them a lot more like account signups and click fraud, and don't want to incentivize the ytdl guys to break it on behalf of spammers/clickfraudsters.