Again, as our motto says here, Hinduism is all about Symbolism. We need to understand what that stands for. We agree that in general, it is most revered animal among Hindus. Until early 20th centuary, cows and kettle is considered as wealth all over the world. Anywhere the grazing lands are found, the cows are considered equivalent to wealth and sign of prosperity. This is fairly common in Africa, Asia, South America, and even in North America.
It is possible that during ancient pre-RigVeda times, cows could have been slaughtered for flesh, but in light of economic status associated with kettle, ancient hindus must have increasingly prohibited slaghter of milk-producing cows. Hindus took "Kettle (cows) - Wealth" equivalence further, they made cows are sacred, they even potrayed God in cows. Cows cannot be killed for food, sacrifice, or any other reasons, making sure wealth grows. It takes years to grow a healthly cow that can yield good milk and other resources, so conserve it, protect it. Philisophically, a good Hindu sees God in everything, and especially in things that benefits lifestyle and growth.
Cows produce milk, dung, urine (गोमुत्र). These three have seen so many different applications in ancient rural India. Considering those, it would be utter waste of such a venerated animal. Almost every Godly form in Hinduism has preferred ride like Indra has Airavat (The mighty Elephant), Ganesha has Mushak (a rat) etc, but cows are kept out of this entire fleet of transportation vehicles. Cows has seen to symbolize a giving mother, like mother earth. On other note, although, cows are treated as secred, the oxen or bull is Lord Shiva's ride named Nandi. Young oxen can also be sacrificed for religious reason.
Ancient Hindu texts are full of cows and their related stories, cows are donated to noble men, cows are given as gifts in wedding, cows are taken as war reparations, cows are stolen, cowherding problems in grazing lands etc, etc. Certainly the cow references are abundent in these texts. Lord Krishna was adopted by a cowherd, he grew up grazing cows. Lord Indra had a wish granting cow named Kaamdhenu (कामधेनु), which led into big anti kshatriya fight of Parashuram.
Subsequently, with the rise of the ideal of ahimsa (“noninjury”), the absence of the desire to harm living creatures, the cow came to symbolize a life of nonviolent generosity. In addition, because her products supplied nourishment, the cow was associated with motherhood and Mother Earth. The cow was also identified early on with the Brahman or priestly class, and killing the cow was sometimes equated (by Brahmans) with the heinous crime of killing a Brahman. In the middle of the 1st millennium ce, cow killing was made a capital offense by the Gupta kings, and legislation against cow killing persisted into the 20th century in many princely states where the monarch was Hindu.