It might be similar to how employers dislike hiring entrepreneurs. People who already have a career bigger than their job
It does not even have to be successful. After getting repeated feedback about my strengths as an entrepreneur and how it is not a good fit for that position, I am now toning that down a lot. YC advertises that funding a company is always good career choice because even if you fail it will be good in your CV. But my experience so far is that many companies see it as a red flag.