Why do humans exist? The basic answer is Evolution.
Chance? No, evolution is not about chance at all. The creature best able to survive continues to exist… there have been many more become extinct than survived, as you would expect.
It just so happens that a creature with a large brain, arranged in the proper fashion, was able to overcome many obstacles over time.
This does not solely apply to human beings. Every niche contains creatures that had the prerequisite abilities to survive a set of variables within an environment.
A Chimpanzee is best suited in its environment, and indeed, may be better suited in that environment than humans are in human environments.
Many ask… but is that all? Is there no purpose? Is there no great plan?
The answer to that is no.
Then what meaning is there to life?
Life has the meaning that you provide it.