There's a limit to how effective companies can be at spending large amounts of cash. Uncontrolled growth can have negative impact. You can't simply make more acquisitions and hire more staff because you have extra money. You have to be as smart about spending $10Million when you have $30 billion as when you had $50 million.
It probably doesn't help that both MS and Apple are sitting on huge (larger than Google's) war chests. This kind of reserve provides a strategic advantage - being the kid on the block that doesn't could be disaster.
Also, billions are harder to spend than millions, because purchases that big change the environment.
For example: a small, unprofitable company makes purple widgets, even though no one else thinks it's worth it to make purple widgets. Owning these guys would give you a competitive advantage because their widget-making technology coordinates well with your sprocket-making pipeline, so you acquire them for $1B. Suddenly a flock of other companies will jump up to make purple widgets; they will get acquired by your competitors, who can learn from your widget-making mistakes. This all cuts down on the competitive advantage that made you buy the company in the first place.
