Nvidia could probably support Freesync quite easily. But their goal is to take away marketshare which is why they insist on the proprietary Gsync standard which requires Nvidia licensed hardware module and makes Gsync monitors significantly more expensive.
At this point it's probably more about pride and trying to steal market share than offering an affordable solution to their customers for the vsync issue.
I think in a few years when Intel comes in to market with their gaming GPUs as they plan to in 2020-2021 I bet they will support freesync. At that point I think Nvidia might be forced to support both formats and support current Gsync monitors and stops putting money in to it and sticks with freesync. Only time will tell but with more players in the GPU market Nvidia may have to cave.