Is Sound Stage an artifact of recording?

A concert hall has a huge advantage.It was expressly designed to have excellent acoustics. Rooms at home are not.

I regularly go to operas and concerts. At times I can pinpoint instruments, especially percussion, but mostly not. 

Creating a "sound stage" on recordings is an attempt to recreate that sound environment at home. It is a nearly impossible task. The best approach to recording orchestral music is to place microphones in such a manner that the "concert hall" experience, not a "sound stage" is reproduced as accurately as possible.

Classical music and operas are unique in this respect. Other types of music rely on electronic effects (both during and post recording) to achieve desired results. Recordings of live performances usually have more energy than studio recordings but suffer from the mistakes that are usual in live performances.

I think you're referring more to "imaging" than a soundstage.

Yes, at a live event the images are definitely more diffuse than what I hear on my system.

Listening in a venue while seated in one location is one thing.

Engineering a recording with multiple microphones in multiple locations is another.

Both respect the music.

It is pleasant spatial cue information stored in the recording. I don’t know why you would label it as an “artifact.” When I attend a symphony orchestra concert, I can clearly hear layering, instrument localization, and depth of soundstage. With proper acoustic treatment in a concert hall to control and diffuse echoes, those spatial cues should be clearly perceptible. Recording and mixing techniques—especially multi-miking—are used to recreate that experience.

Please keep in mind that no concert hall has perfect acoustics. However, some of those imperfections can be mitigated through proper multi-miking and mixing techniques. That might explain why you perceive a difference.  To recap, spatial cues are definitely not artifacts; rather, they are a critical element of recordings.