Many of those sites, incredible for a $4 trillion valued company, are managed by teams themselves on their own infra, thus when there is one of those restructuring rounds that big corps love doing almost every year, some of that gets lost.

Every few years they break all links to The Old New Thing. There is no job Microsoft can't botch.

"continuous development & adaption at a high paced environment"

;-)

This and also due to the fact that positions that involve writing and managing documentation typically do not have great paths for promotions.

Not just at Microsoft, but it's an entire industry issue. It's not a job most Software companies value, so ambitious people constantly leave for better positions and the jobs constantly get moved around to the cheapest cost center where ownership and knowledge gets lost and quality declines.