It’s debatable whether Hindus do worship cows, depending on the definition of ‘worship’. Hinduism teaches to respect all living creatures, but most of us do single out the cow for special treatment, including protection, decoration, rituals of honoring, etc., but not worship to the extent that we would worship a God.
This culture of cow-reverence is an ancient tradition from India, based on the cow’s especial sacredness to several beloved Gods (Aditi, Bhumi, Indra, Krishna, etc.), its being considered an exceptionally pure and Sattwik animal, its enormously nurturing practical benefits to us, and its charmingly affectionate nature when treated kindly by humans.