There seem to be a couple of related issues when it comes to this.
While there are a large number of Korean animation studios (~120), a lot of those studios do largely work contracted from Western companies.
Animated Korean characters have also been becoming more popular in recent years (particularly since about 2010, when the market share was about 28%). By 2014, the market share of domestic characters was 40%, which shows a massive increase. However, that still shows less than a 50% market share of domestic characters, indicating that within the country domestic animation is not as popular as Japanese and Western animation. A few Korean animated shows did become popular in East Asia in 2011 when the animation technique was improved, meaning that if that trend continues we may start seeing more animated adaptions of manhwa when demand increases for it.
Another thing to consider is that manhwa are still being adapted, just not generally into anime/anime-like shows. Instead, K-dramas and movies have been made from manhwa, and their success likely encourages Korean companies to continue making live-action adaptations rather than attempting the riskier shift of switching to animated adaptations. Should there be some major successful animated adaptations, that may change.