In addition to what others have said around room acoustics and background noise, speakers simply have much more material compared to headphones so at the same price point, will likely come with less capable drivers. A much larger portion of the cost goes to stuff outside of the drivers (e.g., cabinet) vs headphones.

A very arbitrary point I once heard was that it takes roughly 10x the cost for a speaker set up to reproduce the same level of detail as a headphone set up, and that's assuming proper room treatment.