A slightly inaccurate, simplification of a 2008 study by the Max Planck Institute for Human Cognitive and Brain Sciences and others, which found that unconscious brain activity can predict a conscious decision up to 7 to 10 seconds before it is made.
But, boring graduate psychology aside, it's a pretty good reason as to why_

FYI_ I can't find the original source of this. Any ideas?
Your prehistoric brain makes decisions in three seconds
We like to think we're rational creatures, carefully weighing options. But the truth is, your visitors' brains are running ancient pattern-matching software, scanning for signals that this is relevant to them. And it happens faster than conscious thought.
This is why clever doesn't work. By the time someone decodes your clever headline, they're already gone. This is why vague doesn't work. If their brain can't instantly categorise what you're offering, it assumes it's not relevant. This is why beautiful design without clarity doesn't work. Pretty confusion is still confusion.
The human brain is lazy. It wants the easiest path to what it needs. Your hero section either provides that easy path or it doesn't.
People won't scroll to find clarity. Well, some do, but not enough, and you absolutely cannot rely on it. They won't give you the benefit of the doubt. They won't work to understand your value. Why would they? There are a hundred other tabs they could click to right now.
And one more thing_
“We humans like to think we are creatures of reason. We aren't. The reality is that we make our decisions first and rationalize them later....Your illusion of being a rational person is supported by the fact that sometimes you do act rationally.”
_ Scott Adams