There are many hardcover AND paperback books with no ebook version. There are many rare books where finding even a library copy is quite difficult! However, as someone who does borrow/buy and scan books, I can tell you that making a quality reflowable ebook, one that is a pleasure to read, with few errors, is a VERY time consuming task! It is not for everyone.
Further, while scanning a book and making a PDF is not terribly hard, taking that PDF and making it into a lovely reflowable text valid ePUB that will convert well to Kindle takes some work and understanding of basic XHTML and CSS. I have used a phone to scan on occasion, and even with good apps, those scans have way more errors upon being run through ABBYY for OCR than I prefer. Which is why I usually buy books so I can cut them and feed them through my document scanner. Or carefully use a wand scanner or flatbed to scan uncut.
It absolutely CAN be done, I am doing it, but it has taken me several years to get to where I feel quite happy with how my books look and perform, at least in the more standard apps/devices such as Kobo, Nook, Sony, Kindle, iPad.
Anyway, I'm all for preserving rare books by making ebooks from them, but it's a hobby few people are going to be keen on. You gotta like re-reading and it helps greatly to be a fast reader, because proofreading is no picnic. Further, preserving illustrations and cover art takes some digital editing skills, you will rarely get hard to find books with great dust jackets.
So, I applaud your zeal, go for it, but don't expect a rush of folks going out to do what you've suggested. Scanning is the easy part, everything else needs devoted work.