[williamstein]

The work of a tenured professor is more than 20% time. A normal teaching load is 4-5 classes a year, plus significant committee work, student advising, etc. It takes an enormous amount of time. And it can be awesome, fun, and many of my colleagues love doing it. But it doesn't result in creating a free open source alternative to Mathematica.

[tjl]

I think people underestimate how much work a professor does. Everybody in my department who started a company either did it before starting at the university or they did it while on sabbatical.

Good luck!