According to Forbes somewhere between 600,000 and 1,000,000 ebooks are published every year in the United States alone.
Imagine being just one of them and not having marketing behind your books? Nothing sells itself.
I stopped marketing for a month, SALES STOPPED. Why? because in most cases if you do not market a product you will not sell that product. I also experimented with pricing. I actually get 10X as many page reads on KDP with a 99 cent book as I do a 2.99 book. Which I thought was weird. But the increased page rank from the extra sales o the 99 cent book probably drives the views needed to get more KDP Page reads.
It’s like any other product in the world. Nothing sells without marketing.