Why Lord Krishna places so much emphasis on Karma?

A popular notion these days is that altruism or generosity is a type of mental compensation. People claim that the 'feel-good' factor is the reason why someone would give to the selfish world. 

If one thinks logically while it may seem paradoxical why someone gives in a world full of self-obsession, vanity, and competition? But in the grand context of life, we often forget that in this huge universe that as living creatures, we are pretty insignificant. Our friends or family members might tell you how important you are to them but after a while, the world moves on and gets over your existence. It shows us that who you are is not that important what you give to the world is. When we depart the world, generally we leave something behind for humanity and mankind. Our work, our contribution, and our kids stay in the world. Our karma stays in the world! 

Many scientists, artists, and writers have perished but their amazing work still survives today. Very few people care to recall the names but they do enjoy the benefit of other's contributions to society. Many great minds have devoted their lifetimes to their work and passion because they believe that long after they are gone, their work will speak and benefit the world. Your work or karma remains immortal, and this is the precise reason why in Bhagwad Gita, Krishna always emphasizes on self-less Karma yoga.