For a lot of reasons. Because the opportunity to learn the skills just fell into my lap. Because I'd read Snow Crash too many times and as a female teenage skateboarder I thought maybe the internet contained my destiny. Because I'd been told there was a world where you didn't need a degree or connections, just smarts. Because it offered me the chance to change my career from cleaning things and running cash registers. Because prior to building my first website I thought the widest audience anything I created would ever reach was whoever wandered through the 4-H barn at the county fair.
Right now I'm building up the dev relations team on a fairly young product. I'm writing a lot of docs, but also reexamining all my assumptions about how developers work with code and trying to look beyond the obvious models of coworkers or open source contributors, which don't quite fit the bill. I'm also dealing with the problem of how to deliver backward compatibility while writing tools and frameworks against a platform that doesn't yet exist.
Don't let the bastards get you down. This is your career, you did the work to get where you are, and the bread that you earn goes on your dinner table. Don't let anyone take that away from you. By the same token, don't let the injustices and microaggressions occupy all of your time. Ultimately, it's a paycheck. When you get some magic on top of that, bask in it. When you get bullshit, remember you're more than just your job.