The "free" version of Vivado is used to develop for Xilinx/AMD's lower tier FPGAs. While offering what I assume are lower profit margins, these lower tier FPGAs make up a large portion of Xilinx/AMD's chip volume.

Xilinx/AMD charging for any of their tools is also a recent thing. 20 years ago, you could download these tools freely without even having to register on their website.