I've been listening to some screenwriter podcasts, and they have an interesting perspective.
Nobody wants to get fired or blamed when a movie fails to make a profit. If they can point to some sort of numbers as evidence it wasn't a bone-brained idea, it covers their ass. They just need numbers to point at. They don't really believe it's going to guarantee anything. I'm sure they expect to lose cash on 9 out of 10 projects, but as long as the 9 duds all came with some sort of numbers, the executive (or acquiring editor in the publisher's case) can blame other factors like bad timing and not their judgement.
So, they're not dumb. They expect 90% of projects to lose money. They're just covering ass with numbers to not get fired.
This also explains a phenomenon some of us have witnessed -- where an unknown screenwriter or in-between wants to partner with indie authors on some screenplay endeavour. The screenwriter wants to use the book's sales numbers to help leverage the material into getting optioned. Sounds like, right now, nobody wants to take on spec scripts that are wholly original and new without numbers from a graphic novel, book, blog, anything.
It's nice, actually, that we indie authors have something of value with our sales. A screenplay based on our books is of much higher value than a screenplay based on no IP. Use that as you will.