You could just as well argue NYC should start to use Central Park for new housing...
Its unfair to compare Manhattan which apart from Central Park is nearly 100% urban to Berlin, which is significantly larger and includes many areas with widely distanced houses and forests.
[0] https://en.wikipedia.org/wiki/Mitte_(locality) [1] https://en.wikipedia.org/wiki/Neuk%C3%B6lln_(locality)