Because I've seen plenty of people try that (myself included) and it's rarely worked as planned. If you've got an interesting idea that you genuinely I think being tied up doing something else for 8 hours a day is just an excuse. You can either fit it in to your down time, or quit completely and be truly motivated to get it done.
Instead I opted to have a long weekend every week, and spend my days doing something I enjoyed for people I liked for an amount of money that let me live a still very comfortable lifestyle in what is reportedly one of the most expensive cities in the world. I've still got a tonne of time to do interesting work, and a family that is happy I'm not coming home complaining about "the office".
>a family that is happy I'm not coming home complaining about "the office"
That was why I quit my last job. I was emotionally invested. Couldn't help it. When you come home and 90% of the time you're complaining to your family about work - it's time to leave.
My current job is a 10 minute walk. 9 to 5. And I'm paid well. I come home quickly and have plenty of time to spend on whatever I want. And right now, it's my family.
Instead I opted to have a long weekend every week, and spend my days doing something I enjoyed for people I liked for an amount of money that let me live a still very comfortable lifestyle in what is reportedly one of the most expensive cities in the world. I've still got a tonne of time to do interesting work, and a family that is happy I'm not coming home complaining about "the office".