Why aren't there enough Police Officers in Downtown Chicago, and Michigan Avenue, especially at night ?

I just moved from Berlin to Chicago about one year ago. When I arrived, there was not much of a problem with the areas of Downtown such as Streeterville, River North and Michigan Avenue. But As time went by, I noticed an increase in African Americans bothering the streets of downtown, and my apartment has warned us about many of them wandering about in the evening and stealing as well as beating up residents around the area. Now, I don't find this a good thing at all. Just this weekend, a man got beaten up by 15 African Americans, and trying to steal his Motorcycle or scooter I believe. This is a popular area of Chicago where tourists go to, all the best hotels are located here and far most, it is a wealthy neighborhood with condos where many young adults and professoinals live in. I just don't understand, why isn't there any law enforcement present in those areas ?... I only see 2-3, and the area is big. Does the Mayor even notice this problem ?