Because the scientists purporting this silly crock don’t meditate, or if they do then they don’t do it seriously. If they did, then they’d get to experience the tremendous depth that the brain has.
We can’t be aware of all of our brain activity. Some of it necessarily remains outside of awareness. This is largely because awareness is a sequential process. You can only experience so many things at one time. Things that you’re not capable of experiencing, because you’ve ‘run out of bandwidth’ are part of the subconscious.
Have you ever had the experience of wondering about something, then moving on to thinking about other things, then suddenly realization dawns on you later? How does this realization work, and what’s happening in the interrim? We need a word to describe what’s going on in the brain. It’s obviously not conscious thought, otherwise we’d be, well, conscious of it. But it eventually feeds into conscious awareness so obviously it relates to conscious awareness somehow.
The positioning here, of the subconscious being ‘beneath’ the conscious, might not be 100% accurate, but saying this aspect doesn’t exist is wrong, and wanting to give it another name is pointless semantics done for obviously political reasons.
I chalk all this up to scientists making overly-provocative statements because it gets them more research dollars and they don’t care what kind of damage they’re doing to people’s self-understanding.