Because they still matter and they produce a different result.
There are a number of routes we could have gone with this project. A simple email with the 100 books could have gotten readers talking. People love lists.
We could have blogged our favorite books and created a couple of ChangeThis manifestos with some clever category slices. My top five books for executives post got a lot of traction and generated some great comments. Josh Kaufman's Personal MBA manifesto has been hugely popular.
Writing a book requires a different level of concentration, both internally and externally. It is not good enough to challenge your co-worker with the five books she'd bring with her to a desert island. Choosing and describing in the right terms 100 titles is a different task. Writing a book gives you the motivation and the impetus to do that work. I am not saying we couldn't do this without a publisher's deadline, just that it helps.
It is the same way a publisher signing the project helps. They sign projects that are going can be commercially successful. That eye can help a book find a larger audience. You also get the sales, distribution, and production capabilities. Those still matter too.
Don't think we aren't going to write the email and manifestos I described at the start as well as a dozen other things. We just think writing a book makes the end result better.