It's not the job so much as the market. There exist people on the market who are willing to put a lot of effort and are able to become very good at what they do. Even if you had a job that gave you half-and-half (20 hours for work, 20 hours to learn, grow, or ship), there would still exist people on the market who are willing to put in a lot more extra effort after the 40 hours.

Those are just the facts of the industry. It doesn't mean there's something wrong with the job, but rather, that there are inevitably going to be some highly-skilled people who got those skills by doing things on their own time.