GNU copyright assignment is to make enforcement actions possible[1]. In the US, copyright law doesn't really give a community much power to enforce their copyright claims. The GPL is not weak because of this, it's just how the FSF operates. The SFConservancy doesn't have this requirement for projects they support, but it's clear that the FSF lawyers believe that having copyright assignment makes enforcement much more powerful.

As for "taking away people's freedom", the copyright assignment explicitly states that your contributions will always be under a copyleft license. So you're not losing any protection of the GPL if you contribute to GNU. That's just an insane proposition.

[1] https://www.gnu.org/licenses/why-assign.en.html