Because managers are evaluated on their ability to grow teams and increase their scope. Organizations intrinsically "want" to hire.

What's unusual about the last 20 years is that there's been so much available capital to fund tech companies doing just that.

Growing your office plankton messages to investors that "we are growing, give us more money".

There is constant pressure from upwards to "increase scope" and hire more people.

The same reason a product is never finished and there is a never-ending churn of "features" nobody wants or needs.

The bureaucracy expands until external factors force it to stop or contract. Google Search and Ads are a small percent of the company but nearly all the revenue. It’s not hard to see how that can lead to runaway bureaucracy.