Why not call each sect in Hinduism as its own religion?

Sects within religions are not uncommon. Almost all world religions are divided into multiple denominational families, and there are subtle differences in their philosophies. Although this is the case, it is not difficult to infer that they still belong to the same religion, quite simply because of whom they consider as the Supreme Being. Simply put, all sects that believe in the same God comes under the same religion. In the case of Hinduism, the sects do not believe in the same God. For instance, Shaivism accepts Lord Shiva as the only Supreme Being, whereas Vaishnavism believes only in Lord Vishnu. And of course, there are yet others who consider their own Gods as the only Supreme Being. 

Another point is that other world religions follow the same religious text. Again in Hinduism, the texts accepted by one may not be accepted by the other. In fact, there are objections and strong criticisms of these writings by their rivals, not to mention the differences in their philosophies.

Yet, why are they still considered as one religion? Could it be because of geographical reasons? In one of the earlier articles, the origin of the term Hindus and Hinduism being related to River Indus was discussed. Could it be said, any religious belief that had its origin in the Indian sub-continent regions be classified under Hinduism? Interestingly, there were/are several religions that had/have their origin in this region but are clearly defined as their own religions. For example, Buddhism and Jainism are a couple of those, which are still widely practiced in India and around the world, and they do not come under Hindusim.

So the question is, on what basis a religious sect is considered to be under Hinduism?

To answer this, let us consider all the religious sects under Hinduism and analyze. Although these sects have their own religious texts, none of them claim these to be their original book. Neither they claim to have established the philosophy. Invariably every single sect mentions the time-immemorial practice of the said philosophy. Most importantly, they all fundamentally accept the world's oldest scriptures - The Vedas.

Banyan trees are characterized by several aerial prop roots. As years pass by, these prop roots mature into thick, woody trunks, which merge with the primary trunk and become indistinguishable. Just like how the banyan is one tree despite its several props, Hindusim is one religion with many branches, and the Vedas is its primary trunk.

According to the Vedas, the Primordial and Supreme Being is Brahman. With respect to the creation, the same is called Iśwara. Shaivites say Lord Shiva is Iśwara and thus Brahman, while Vaishnavites say that is but Lord Vishnu. In other words, the definition of Iśwara and Brahman is the same and is as specified in the Vedas. Only the name is changed. One says Shiva, the other says Vishnu, yet another says Shakti, and so on.

It is now evident that all the denominational families in Hinduism accept Brahman as the Supreme Being and Vedas as their fundamental text. Thus any religious sect belongs to Hindusim as long as it accepts Brahman and the Vedas.