The hell? People change jobs for a million reasons, few of which have anything to do with "back" and whatever macho/nationalistic fantasy you've got going on there.
- People at the beginning of their careers sometimes improve their skills more rapidly than their employers can accommodate. eg, the guy who starts out doing desktop support and grows into a sysadminning role, at a company that's already overstaffed on sysadmins.
- Companies downsize or go out of business. Any time you join a startup it is a crapshoot (mostly based upon factors outside your control) whether it will still be around next year. Does that mean that no one should ever join new companies?
- Many, many people simply cannot afford to live anywhere near their offices.
- Changes in medical conditions may alter the type and amount of work that you're capable of.
- Changes in your or your family's medical or educational situation may alter the amount or reliability of money necessary. eg, moving to a less fulfilling job at a big corporation with solid medical benefits.
And, frankly, change and drive and curiosity are good things. I would much rather hire someone who has displayed the ability to excel in ten different environments than someone who has sat still at one company for a decade.