I have thought about it a lot, and I think the main reason is that there is no single doctrine / school that everyone agreed upon in the past. I see this is as the pinnacle of free-thinking. The seeming consensus among people who follow Abrahamic religions has provided the functionality to conserve itself in some tangible form.
Whereas the meaning of Hinduism is just lost, because there is no religion single religion as such. Nothing comes close to the deep thought and inspection that philosophers (sages/rishis whatever you want to name them) of the ancient India. All their "work" has been hidden under layers of what people assume is just religious "stuff".
Carl Jung, Nikola Tesla and innumerable people have invested time in discovering their true essence.
The concept of "religion", "spirituality", "god" carry a lot weight, especially in the modern world. I myself (being a Hindu), assumed that all that was present in Vedic texts was just references to god, different kinds of gods, different rituals etc. And as a modern day human, I just assumed that these are just things of the past and now we are evolved enough to ignore these concepts and move towards the future.
And I was proved entirely wrong when I started reading The Upanishads, which are part of the Vedic literature. The concepts are so abstract that different schools/sub-religions spawned just on the basis of different interpretations. Roughly dualistic and non-dualistic interpretations.
I can go on and on about my discovery. In fact I am writing about this here: https://pulkitsharma07.github.io/2020/06/25/source-0/
I am really happy to see this post on HN, hoping this doesn't get flagged.