They are so enormous that they stifle themselves. Sometimes Microsoft seems to put out something, from some division, that makes one think: "Hey, this is a good move -- maybe they're starting to get it." But before one can utter those words, another of Microsoft's many appendages does something to completely nullify those sentiments.
That "appendage" is usually the head itself. Microsoft often acquires a good thing (many times a catchup move) but this thing only stays good for a short time. When integration is complete, it's a single bumbling Microsoft entity once again.