Sapkowski doesn't/didn't believe that video games are a worthwhile medium of telling a story.

When CDPR wanted to make the first licensed Witcher game, he wanted a one-time payment of about 10k USD or thereabouts because he didn't believe they'd be successful. When they made it big with The Witcher 3, he sued them for their profits, claiming that the licensing contract only covered a single game. They settled the lawsuit, and I think they have a new contract that gives Sapkowski part of the profit.

Dmitry Glukhovsky, author of the Metro novel series, publicly called him an old fool for this on the dead bird site. He's probably right, the games are what propelled his books to worldwide fame.