Youtube has all the buttons viz Upload, Browse, Subscribe, etc. but it doesn't come with an inbuilt 'Download' button. Viewers have to use third party software to download the videos from Youtube. I wonder why.
Because the premiss of the site is that you go back to view your favourite videos so you can see (more) advertising, sponsored links etc.
If you download the videos you could watch them as many times as you wanted without seeing all this.
There's also the small matter of copyright - you only have the right to view the videos, not copy and (potentially) redistribute it.