1. I write software: business applications. I love what I do. I love getting something to work right the first time. I love seeing people use the software I wrote to do their jobs and run their businesses. I can’t imagine doing anything else.
2. The software I have inherited in all 88 companies I have worked at has sucked. I mean really sucked. Nothing to be proud of. Nothing to want to work on. I think it’s because business software is now where medicine was 100 years ago.
So I have a choice. Work on other people’s crap or write my own. I have done both, but I have to write my own to be happy in this industry. If I could only work on other people’s software, I think I’d rather work in a grocery store.
Starting a software business is the most direct way to do what I “really” want to do.
I realize that other people have different reasons; this is just one answer to your question.