Because the more you practice the better you'll get. It will likely not make perfect since very few things are perfect - except for the Bailey's Irish Cream Cheesecake at the Anchor Inn in Round Pond - but I guarantee the more you write the better you will get.
It's just like playing a sport or an instrument. You cannot become proficient unless you practice. Why do you think they have practice after school for band and field hockey? Because if the band is going to be any good they've got to practice together, learn their craft. Likewise the field hockey team. There are rules to the game, ways to work to gether successfully. And for you and me? We have to write and read and study others who have been successful at our craft in order to see and understand what is good and what is not.
As an example I give you Terry Pratchett who writes the Disc World series, which I like to call satirical fantasy. His first book was fun - if not great - and he went on to write over 30 more books in the series. However, two of his more recent books in the series, Monstrous Regiment and Night Watch, are practically brilliant and it was while reading them that I saw just how much Mr. Pratchett had improved his game.
Yes, you have to have some innate talent, I suppose, and if you're writing fiction an imagination is definitely a requirement. But after that, well, all you have to do is practice.
Because while practice may not make perfect it does make pretty damn good.