Did I mention that unlike speakers in the house, or full sized headphones, you can hear stereo when you lay down on your side with your IEM's in both ears.
Maybe IEM’s are a step down from high end cans costing tons more, but those same cans are a big step down from moderately priced real outside of your head speakers.
When music is recorded you can hear it in both ears from both sides all of the time. Cans as well as IEM's, and buds, separate the sides completely. This changes, distorts what was there before.
Full sized headphones aren't stereo? Do you have a grasp of what stereo is? It's a simulation of how music doesn't emanate from a single point source but instead from a sort of 3d plane, thus by having two channels, you widen what is initially a point source instead to a more diffused plane source.
I'm not sure that you have had much exposure to high end cans compared to IEM's. Personally, I have a pair of $40 DIY headphones which slaughter the IEM's that I've heard (in everything). My primary headphones are considerably better than my DIY pair and compared to even some of the best speakers (for example: Quad ESL989's with a sub, can't remember the sub) they have better detail retrieval (though I did enjoy the Quad's more).
You're also mistaken here. Have you ever heard of binaural recordings? These are for strictly headphone/IEM use. Look it up sometime. Also, I use a passive crossfeed circuit which uses time delay and by blending both channels a little bit in order to emulate how it would sound if played through speakers.