Because it's time limited. National lab postdocs are valid only upto 5 years from PhD. Many grants won't fund you if you are X years since PhD. That's why the

Because it's time limited. National lab postdocs are valid only upto 5 years from PhD. Many grants won't fund you if you are X years since PhD. That's why the position of Research Assistant Professors exist - the way around the postdoc time limit loophole. However, RAPs are significantly more expensive than postdocs, which is why faculty are more eager to phase out senior postdocs and hire fresh blood rather than convert them to RAPs.