Because it causes burnout. You can't write open-source software as your primary day job well. There's too much risk. You can, however, do it as a function of y

Because it causes burnout. You can't write open-source software as your primary day job well. There's too much risk. You can, however, do it as a function of your work for your employer. Almost all of today's viable FOSS started like this, because it provides the financial security and moral support you need to bring a FOSS to maturity.