TL;DR People are pushed to specialize as companies grow and meetings are cartesian products of interests.

As responsibilities become increasingly important (more financial import), their work is divided into specialized slices for optimization, redundancy, and bus factor. The number of combinations of people that might be necessary to answer questions or provide guidance in decisioning expands exponentially (small slope, correlating to recruitment). If something in a meeting is considered missed, unsaid, or backtracked, the event is memorialized and pushes for more meeting with a larger audience. The people in large meetings self-select into smaller meetings around their specific vertical interests, in order to align with the interpreted decisions of the larger meetings.

Meeting bloat is the result of a conflux of human behavior, lossy communication, optimized behavior and competing interests. If it was a simple single reason, humans would have solved it long ago.