Why there is a politics in the office? What would be the reason for employees to do politics in a company?
Can you expand your question a little, Jupiter?
Actually this is blaming each other for not performing the assigned task. Due to non performing one who is showing his hand and saying that because of his failure I am failed.

The other type of politics is asking the same question with two person, finding fault, cornering the right person and make him de-motivated indirectly.

Offering the hire position for a person who does not know the basic knowledge of work/job, which creates confusion on the day to day work.

Hope that the above will clarify your doubt.
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That is a complex question, but it boils down to we are all human. Most people are working to earn a living and will do what it takes to sustain that life. They are not working for the benefit of the company in the end, and so enters the politics. Instead of what is best for the company they do what is best for them... that spirals into unions between employees, poor decision making... Even worse, when a leader position his friends in ciritical positions (very common). Imagine an environment where only frienships and allies matter, and business does not. It is not sustainable, but for the short term it makes no difference.