Because it takes most people a number of years to learn this career to a point where they can grind a profit from the markets on a constant month by month basis. I feel there is nothing wrong with this, and from the general consensus of this forum and others it's not an exaggeration, either.
In my view, which is quite often different to others, I see no point in going straight into a live account, or even after gaining some basic understanding either. You're bound to lose everything at the start, that is the learning curve after all. So it's better to do this with 'demo money'.
As explained in this post, demo should be extremely similar to live - if the degree of difference is causing you to have sever variations in your results then you need to look at your trading, not the account.