It is surprisingly difficult to say which closed ways are areas and which are not. This depends entirely on tags of the way and is only solved by heuristics.
In addition, it is common to have objects that are both area and line at once. Or area according to one tool/map/edtor and line according to another.
And many, many multipolygon relations are in inconsistent state and require manual fixup.
Also, complexity of entire area baggage makes explaining things to newbies more complex. You can either try to hide complexity (used by iD in-browser-editor) leaving people hopelessly confused when things are getting complex or present full complexity (JOSM) causing people to be overwhelmed.
The current format stores locations and references to locations, so for example, a line feature only stores references to locations, so to realize it on a map, you have to go through the data and find all the locations it references and build up the actual geometric feature. So people do caching and so on, for sure.
The proposed changes would make that sort of data transformation easier and less resource intensive.
https://github.com/tyrasd/osm-polygon-features