Wow, I would've actually guessed more like 95%...I think the value of domains has dropped over the past decade, so people are less willing to pay for dormant ones.
I think most aren't due to people trying to sell. Most registrars put up a parking page automatically, and many (myself included) buy domains with the idea that it'd be a cool site and never circle back (and have auto-renew on).
Yeeeah.. I finally let go of a handful of “haha this is great. How was this not already taken?” Domains at the behest of my wife lol.