Why does OpenAddress need to exist separately from OSM? Can address geocoding data not be properly represented in OSM?
Many addresses exist in OSM. But there's a preference from the OSM community that data be entered by users familiar with their areas, that they can verify on the ground. Imports have happened in OSM, such as the controversial TIGER import in the US years ago, and the NYC building dataset more recently. But these require a lot of effort and attention to manage well.
I've heard many folks push for consolidation of various open data projects, so they aren't "competing" for contributions. However the reality is that they are currently very different and each serves to bridge a very specific gap.
In the meantime, it makes a lot of sense for projects relying on open data to pull all of these sources in, as Mapzen Search is doing, to ensure the best possible coverage.