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.
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.
See https://wiki.openstreetmap.org/wiki/Area#Tags_implying_area_... for a start of a complexity fractal.