I have pet projects. I also have a life. I have a job that involves putting my knowledge/skill/passion to work to a degree. Even if my employer would allow me to write each project in whatever programming language I was obsessed with that week, I wouldn't, because I don't want to maintain all my whims. Personal projects can be thrown away easily or restarted. All that work can be done without _any_ business concerns. That's just not reasonable for a paying job.

I have a lot of passion for programming and I hack it quite well in a professional environment, thank you very much. My passion requires that I fiddle about with personal projects so I can test out new languages, frameworks, styles, libraries, paradigms, etc. without any business concerns in the least. If there is a place where I could get paid well to just hack on whatever the hell seems fun at the moment, I'd definitely make a concerted effort to work there--I don't think it exists.