Being self employed I always wonder the same thing!
What keeps me staying self-employed is the other benefits - more flexible time, control over workflow, as a designer i was frustrated by bosses having creative control over my work. Having the freedom to work from home. Also the enjoyment of seeing a company grow, and getting to choose what way you want to move the company forward.
For us its been absolute blood, sweat and tears, but very rewarding. I found working for a company very stifling and frustrating. I hated not having control of my time, and even though I work weekends, nights and most holidays, i still love the freedom of being able to swing an hour free to meet a friend whenever i like. I like having the freedom to put work on hold to do that night, so i can do my hobby during the day for a few hours.
I love having nice clients and developing a relationship with them. I like the freedom to price jobs myself. I like being proud of the work we do 100%.
I found i just coasted along in my last job, even though i worked hard. I find the challenges of running a business fascinating, learning admin, pricing, promotion etc. It constantly challenges you and I learnt more and matured more in one month of being self employed than i did in 10 years in a job.