Here's the key: when businesses develop dominance in a market, they get fat margins - and they often just 'hire'. It becomes instinctive to think that 'people create value' and so they hire. Moreover - VP's, Directors, Managers are all trying to build their status, and they 'push to hire'.
I worked at a post-startup-stage high-growth company, and my VP was given directive to 'hire 100 people'. Completely not driven by business needs, or project needs, or ROI. 'Just hire'.
There is sometimes good strategic reasoning for 'just staffing' up i.e. scrape all of the talent in an area way from competitors, be a 'real company' to validate the upcoming IPO etc.. but it's really perverse.
Google, FB, MS - all guilty of this. Regular companies just the same.
I'll bet 1/2 of finance today could be replaced by computers - or frankly nothing. 1/2 of Hedge Funds could just put their money in indexes, drop their staff and get the same results.
Large successful companies often spend billions on 'new projects' that are doomed to fail - one could argue that 'failure' is part of the innovative process, yes, but it might be more efficient to invest/partner with startups.
When big, lumbering companies start to wane, well, they can't afford all the graft and have to start letting go.
It's an odd cycle, and it's surprisingly not as directly related to productivity as we might want to imagine.
I once worked with a large internal marketing team of maybe 25 people, they did almost nothing. Nobody was really sure what they were there for. They 'did stuff'. They were smart, nice. But they were very irrelevant - and I took pains not to discount their effort, i.e. not realize that they were 'important but unrecognized' cogs in the wheel. No. They were hired 'for hiring sake' and 'worked' for the sake of 'work'. This is common. A lot of big, entrenched industries are like this.