You can consider the GPL a contract, but consider the hypothetical situation: I download a GPLed software, implicitly accepting the license/contract. Then I violate it by giving you a copy of the software without making you accept the GPL.
If the GPL is just a contract, then you can now distribute the software as you wish, since you're not bound by such contract. Copyright, on the other hand, doesn't work like that: everyone is bound by default, and you need a license like the GPL to "break free" from copyright's restrictions.
If the GPL is just a contract, then you can now distribute the software as you wish, since you're not bound by such contract. Copyright, on the other hand, doesn't work like that: everyone is bound by default, and you need a license like the GPL to "break free" from copyright's restrictions.