I keep hearing that there is a serious shortage of Security personnel. it is a no brainer that there is certainly demand for security professionals now a days. But what I don't get is the reason why so many experienced infosec guys and gals keep on piling up security certifications. Is it for significant raises? For promotion to become boss of boss?

For jumping into a different security stream? or is it because there is a glut of infosec professionals and one needs to stand shoulders taller than others?
Like for example after gaining CISSP why struggle for CISA or CISM?
Please enlighten (and encourage) me