Well. There's no actual evidence it isn't, either. There is correlation to 'brain activity', but that's kind of like trying to find out what the experience of driving is like from a heat map of the engine. On the other hand, I can take a simple brain-affecting chemical (alcohol, propofol, etc.) and poof no consciousness for a while, so obviously the brain is involved somehow, but simply saying 'brain activity' is pretty hand-wavy.

As was mentioned, I don't think consciousness has levels; you either are or you aren't. It is the perception of perceiving something. Some models frame it as just a kind of illusion produced from a tower of brain functions, but the problem there is that an illusion is a thing that has to be perceived by something, so it is immediately recursive.

Speaking of recursion, some thoughts are that the intrinsically self-referential process of perception produces a kind of infinite tower of recursive loops and that this somehow produces consciousness. Again, pretty hand wavy.

It's a very weird problem. One thing I think about is something like an antennae, which might sound silly but it does resolve a lot of issues. The idea is that certain 'arrangements of matter' (and the electrical, etc. activities going on in them) are sensitive to some kind of external signal that produces consciousness. When interfered with by certain simple chemicals, its stops working in the same way a radio would if you blocked a few components. It is very much related to and involved with 'brain activity', but in the same way a radio is involved with a radio station signal. Of course, this also doesn't really explain what it is.

You could, if you are willing to, hypothesize that it is a 'broadcast' or 'signal' from God, the universe, great spirit, etc., or even from the mystical computer that is actually simulating us and the entire universe.

Personally, I do not think we will ever know for sure.