Why? Because the world is more complicated than the FSF ideologues want it to be. Developers do not collectively or exclusively want to work for free and live a life of poverty.
Not to mention that the GPL is simply too risky and expensive to use in any commercial context. There are countless lawyers consulting organisations on how to avoid the viral aspects of GPL-licensed code. And since case law is scarce, more often than not the advice will be: avoid the (A)GPL like the plague.
The only meaningful and morally acceptable “copyleft” license is the LGPL, which asks you to give back without being the license cancer that is the (A)GPL.