Humans tend to like to stay at there they are, in the tech context especially. It takes a long time relative to the length of a career to learn new systems, make new connections, and achieve some level of independence at a new job. There's also relatively few jobs that at least pretend to be somewhat beneficial to the world while paying somewhat competitive salaries (I can't think of any that I've worked at). Then there's relationships you may have developed with your colleagues.

Finally, if everyone just leaves their jobs without trying to improve them, won't everyone run out of places to jump to eventually?