Because the two species are so closely related. Humans, chimps and bonobos descended from a single ancestor species that lived six or seven million years ago. As humans and chimps gradually evolved from a common ancestor, their DNA, passed from generation to generation, changed too.
Human & Chimp DNA Similarities. A high degree of genetic similarity (as high as 99% according to some studies) between humans and chimpanzees leads evolutionary biologists to conclude the two species share a common ancestor. However, a closer look at the genomic data presents a less certain and far more complex picture The chimpanzee–human last common ancestor, or CHLCA, is the last species shared as a common ancestor by humans and chimpanzees; it represents the node point at which the line to genus Homo split from genus Pan.  Genomics and the DNA Revolution. One of the main problems with a comparative evolutionary analysis between human and chimp DNA is that some of the most critical DNA sequence is often omitted from the scope of the analysis. Another problem is that only similar DNA sequences are selected for analysis.  So Much Alike... Human and chimp DNA is so similar because the two species are so closely related. Humans, chimps and bonobos descended from a single ancestor species that lived six or seven million years ago. As humans and chimps gradually evolved from a common ancestor, their DNA, passed from generation to generation, changed too. In fact, many of these DNA changes led to differences between human and chimp appearance and behavior. © AMNH Exhibitions. Chimps and humans are taxonomically close to each other, but there are enough differences between them to distinguish separately. Despite many similarities between human and chimp, the external appearance and behavioural differences are prominent. Chimps. Chimps or chimpanzees are a type of apes and the closest extant relative to the humans. Gorillas and orang-utans are their other close relatives. There are only two species of chimps described under the genus Pan, P. troglodytes (Common chimp) and P. paniscus (Bonobo). Chimpanzees, on the other hand, are actually two species: pan troglodytes, the common chimpanzee, and pan paniscus, the gracile chimpanzee or bonobo. These two types of chimpanzee are completely separate species. Humans and both chimpanzee species evolved from a common ancestor, possibly sahelanthropus tchadensis, between five and seven million years ago. Only fossils of this ancestor remain The anatomical evidence-both fossil and contemporary-demonstrates that australopithecines and chimpanzees share a geologically recent common ancestor and that Homo sapiens are descendants of the evolutionary branch that began with the divergence of the australopiths.  King compared the genes of chimps and humans. When she did, she was in for a surprise. The humans and chimpanzees were not 50% similar genetically, or 60%, or even 80%, they were 98 to 99% similar, nearly identical. All of the differences between us and them, must relate to the 2%. What has followed has been a rich and detailed consideration, a consideration that is still very much underway, of the 2%. As a results, humans and chimpanzees are really no longer that similar. The differences in the species we associate with, our parasites and pathogens (and then also our agricultural species), are the biggest differences between us and them, between
http://www.reasons.org/rtb-101/humanchimpdnasimilarities https://en.wikipedia.org/wiki/Chimpanzee%E2%80%93human_last_common_ancestor http://www.icr.org/article/human-chimp-similarities-common-ancestry/ http://www.amnh.org/exhibitions/permanent-exhibitions/human-origins-and-cultural-halls/anne-and-bernard-spitzer-hall-of-human-origins/understanding-our-past/dna-comparing-humans-and-chimps http://www.differencebetween.com/difference-between-chimps-and-vs-humans/ http://listverse.com/2012/02/14/10-comparisons-between-chimps-and-humans/ http://www.antiquityofman.com/ape-human.html http://www.yourwildlife.org/2015/04/chimps-and-humans-are-less-similar-than-we-thought/
similarities between humans and chimpanzees