A big reason is because it's not possible on the web, and it's worth noting that the kind of visual you see in masonry demos is the most extreme example of what

A big reason is because it's not possible on the web, and it's worth noting that the kind of visual you see in masonry demos is the most extreme example of what you could do with it. Most of the time it's: I want to fill a page with modules of differing heights where positionality is based on importance not coordinate, without weird gaps where items fall into the same row and constrain each other.