I think people believe it is a sign you are striving to excel or that you care about the work you are doing.

In my case I think I talk about how much overtime I work because I got insecurities about my productivity drilled into me as a child with undiagnosed ADHD. Constantly being told you don't work hard enough regardless of the effort you put in will give you some weird hangups. I think subconsciously its about needing external validation that the time you put in was adequate, or insecurity around 'work ethic'