For me the biggest difference between headphones and earphones is the interaction with your pinna and HRTF. Headphones directly have the sound interact with your pinna and changes the Frequency response to how you're used to hearing, where as IEMs Bypass your pinna and use an approximation of an average pinna gain.

That said, yes earphones are very good, and there is a very noticeable difference between a $50 pair and a $300 pair.

My wired IEM earphones are $320 and there's quite a difference in sound presentation with that and a pair of over ear headphones that cost the same. Both excell in their own ways.

Our perception of sound is also heavily influenced by other factors. Fit, space, how much sound is let through. There's so many factors that change not only the frequency response, but also our perception of sound, which makes no pair of headphones, even of the same model sound exactly the same