The throttling is done client-side, yes. But it's a numbers game - if you enable "official" direct unthrottled downloading, this would bypass the client throttl

The throttling is done client-side, yes. But it's a numbers game - if you enable "official" direct unthrottled downloading, this would bypass the client throttling, effectively increasing network requirements. Of course, this is just a side point - the strongest reason is likely to be either that YT has a strong incentive to keep users in the site/apps for ad display and engagement (related videos, etc), or that they are unable to do so due to some sort of business deal. My opinion is that - even if they're not bound by any deal that would forbid them to do so - they would not officially support (much less encourage at any level) direct downloading, because the ads/engagement reason alone is strong enough.