Here’s a rational answer to this question.
According to Hinduism, what is divine is not God, or Heaven, but human life itself. So we must celebrate life. Every day of our living life is a special day. We celebrate life by acknowledging all the things that make us live our life better. These could be forces of nature, for example, the earth, the sun, the rain, and even plants and animals. So we have a Sun God, a Moon God, Rain God, etc. etc.
Also enabling us to live our lives well, are the special people around us: our parents, siblings, friends, grandparents and extended family, even our teachers. And so we have special days in the calendar earmarked for each of these special people – for example Ancestors’ Day, Brothers’ Day, even a Teachers’ Day.
With so many forces to acknowledge, it is no wonder we have so many “festivals”.
Do we really worship cows? In reality, we worship all animals. The cow is merely a symbol of all animal life. Just as Nagini (yes, the snake from Harry Potter!) is a symbol of all reptile life, and Garuda (the name of Indonesia’s airline) is a symbol of all bird life.
We might as well answer that other question Hindus are often asked: why do you believe in reincarnation? ‘Reincarnation’ is really an over-simplified way of saying that if you lead a good life, you will get another chance on this earth. Notice we don’t say you will be rewarded with heaven: because Hindus believe that if there is a heaven, it is right here, on earth.