Where I worked previously it wasn't generally common for developers to have a side project. It probably depends on where you work. I presume everybody in Silicon Valley plans to start a startup like every waiter in Los Angeles plans to become an actor.

One reason I can think of, is maybe because they can - developers have the means to create software themselves though maybe not a way to market it. Anything physical requires a fair bit of capital to manufacture it, where's developers just need time. I suspect also that independence is more important to "developer types" than to others.

Another reason I can think of is management in companies can tend to treat developers as second class citizens which can make work unfulfilling. Working on your own products can make programming fun again.

Also in a lot of other white-collar areas like design, medicine or law it's fairly common to setup your own studio/practice after a few years of work experience. IT tends to create large global companies which sucks in a lot of developers.