This already exists just not as DTube, DLive is a live streaming site using Steem made by a completely different team.

While DTube could make their own, one of the advantages of being Decentralized apps (Dapps)based on a common backend (Steem in this case) means that two apps can do there own thing and still be compatible. So for instance, you could livestream to DLive and upload non-live streams to DTube, both would get posted to your Steem profile as a combined feed.

DLive also supports uploading videos to their site instead if you really want a combined all-in-one site like a YouTube but personally I think it's better to have two inter-compatible sites. As the saying goes, "It's better to do only one thing and do it well than to do everything and only do it decently". That's the exact issue YouTube has right now, they are trying to do everything and are failing hard because of it (aka why many people stream to Twitch even though YouTube LIVE exists).

tl;dr: DLive already exists for livestreams, DTube doesn't want to do it since DLive exists and can work together with it since both use Steem.

PS: DSound also exists for audio uploads like SoundCloud, also another completely different team separate from both DTube and DLive.