Because before your know it your very much open source company called Prusa will end up being competed with by subsidized stuff from the east based on your own work.
So what? They make something better, so I take what they did and improve my product.
It's not my problem if someone from the Big Red East does something with my work as long as they share it too.
They also don't always do that. Look at the super popular BOOX e-ink notebooks. 100% in violation of the GPL, and has been for years, yet available to buy in common retailers such as Best Buy.
These are edging out popular devices, such as reMarkable, which are GPL compliant.