For license on software, users who download the software must abide by the license, usually in something like a LICENSE file. I don't need to ask them to fax me a signed document or anything.

For CLAs, I noticed that it's somewhat common for projects to require you to email/fax them a signed document.

Why can I not just communicate that policy with the CONTRIBUTING.md by saying this?:

By submitting contributions, you agree that you are only submitting contributions that you have authored and you agree to assign copyright of those contributions to [project owner] so they may adjust the license of the project in the future, if needed.