Because the Essence that is the arrangement of molecules replicates onto other molecules, and it is more effective at replicating when it dedicates some of it's molecules to perceive and model it's own existence, in order to run experiments on it. That way it's more likely to find a behavior for the rest of the molecules to successfully replicate. So the molecules experience themselves, and the experience becomes more intense the more advanced the experiments are that can be run on the model.
There is an "Objective Reality" (mutable, evolves into many forms and includes both material/non-material concepts) and a "Witness-Consciousness" (immutable and attributeless). The latter is embodied in one form of the former which we call Sentient Living Beings. But we forget the distinction between the two and experience reality through its own subjective evolutes within the embodied being i.e. Sensory Mind, Intelligence and Ego. Various techniques/practices have been prescribed in Samkhya/Yoga/etc. to break out of this illusion and realize that one is simply pure awareness/consciousness beyond any experience. This is what is called as Kaivalya/Moksa/Nirvana i.e. a state of mind in which there is total freedom from everything to do with objective reality.
See also the paper A landscape of consciousness: Toward a taxonomy of explanations and implications and previous discussion on it here - https://news.ycombinator.com/item?id=40844824
Also see the classic Yoga and Western Psychology: A Comparison by Geraldine Coster - https://archive.org/details/in.ernet.dli.2015.189086/page/n1...
It's not true that we have to be conscious - we can do things on autopilot too. The same way we can be thoughtless in treating others - we can turn the machinery off.
Recently I was thinking of a very hard problem and I cannot absolutelly recollect approx 30 minutes of the drive home. Apparently my body and some part of the brain took over the driving, while my cortex was busy with the problem reserving the short term memory completly. I snapped out of the situation when apparently a stronger break was needed, and this intrerupted my flow and popped my attention to the road. Fascinating!
That part of the brain is the cerebellum (mostly).
The Minds That Left Reality | Diaspora
I do feel like most forms of leisure - watching / playing something, alcohol / drugs, sleeping - are basically this, you lose some of your consciousness and awareness in favor of what you're focusing on, or floating / free associating.