the ability to de facto say "by using this software you agree to ..." is based on copyright - because anybody can refuse the license, saying "by using this you agree to X" is technically a lie (by the way, by reading this comment you agree to send me $1000 worth of BTC within 10 days), "using this" does not make you enter a binding contract, it's just that if you refuse to accept the license, you're infringing on copyright. The licenses are accepted because of threat of a copyright infringement lawsuit. If there was no copyright, people would be free to ignore all such license offers as they would not require a license in the first place.

Indeed, this is stated quite explicitly in the GPL itself:

> ...nothing other than this License grants you permission to propagate or modify any covered work. These actions infringe copyright if you do not accept this License. Therefore, by modifying or propagating a covered work, you indicate your acceptance of this License to do so.