The bunch of different lat/lon datums we have is a relict of the first surveying done on national basis. These had their limits on the borders (mainland or overseas colonies), and noone thought of putting a universal CRS at that time. As a consequence, getting lat/lon coordinates always needs a question about the datum they are in. While local projected coordinates can easily be separated from their extent, it is much more difficult in lat/lon systems.
Tectonic moves are yet another reason against using universal lat/lon coordinates.
The plates have already moved since WGS84 was established. That's why the Europeans put up their ETRS for surveying restricted to their tectonic plate. As a consequence, WGS84 was ammended by time frames to fit to ETRS. You can think of that as a
datum on a time axis rather than locally restricted like the old datums were.
Up to now, it has not yet come to the attention of many GIS people, but those living on the edges (like on Hawaii) think different about this.