Because it's not an Elo system, but a Glicko one.
In a Glicko system, there is a measure of how certain they are about your rating called RD (ratings deviation). The higher the RD, the more your rating is likely to change. When you first start, you'll have a high RD, but as you play more games and finish them regularly, your RD is likely to go down as the system becomes more and more sure that your rating is accurate.
On lichess we start from 1500.
That's not true at all. On LiChess, if you haven't played any games, you are considered "unrated". That's how it's supposed to be!
and provisional rating for first 20 games in each category