Stereoscopic 3D: a two-image illusion
Stereoscopic 3D never actually reconstructs light the way a real object would emit or reflect it. It sends two flat, offset images to two eyes and relies entirely on the brain's own depth-fusion process to create the illusion.
True holography: reconstructing light itself
A genuine hologram records and reproduces a full light-field — the actual pattern of light waves that would come from a real object — so it looks correct from any angle, to any number of viewers, without needing two separate eye images at all.
Why holograms remain rare outside labs
Recording and displaying a full light-field at useful resolution and size is an extraordinarily hard engineering problem. Most 'hologram' displays marketed to consumers today are actually stereoscopic or light-field approximations, not true holography.
What's realistic near-term
Stereoscopic 3D, spatial video and light-field displays like Looking Glass are the technologies that will keep advancing and reaching consumers over the next several years. True consumer holography remains a longer-term prospect.
See it in action at 3dstreaming.org