Because the construction of speakers and microphones is a very simple thing. Most "cheap" speakers are "good enough" that most people cant tell the difference. When you pay $99 for an SM57 you know you are throwing ALL of that money in the garbage can because the mic itself only costs $2.57 to make. You are paying for that "sound" and hopeful that their is some good quality control.
There is no valid engineering reason someone else cant make the exact same mic and sell it for $10 and make huge profits.
Money does not equal best sound.
Now once you start adding in wireless etc, things can get a little more complicated. I agree.
Cheap earbuds are not made to play at the spl levels needed. Even molded buds only blocks out 25db of ambient sound. This means that eventhough we might not be aware of it, we play much louder in earbuds when on stage, than if just walking the dog.
So no, cheapo socalled iem (everyone will call even the cheapest of earbuds an iem) just wont do. They will crap out and sound terrible, because they are being maxed out. There is a reason companies like Ultimate Ears and Vision Ears excist.
Add to that, that there is a pretty big difference in what sounds good on an entire mix playing from your mp3 player, and what feels comfortable for a guitar sound. The most hifi sounding buds are not very suited for guitar playing. You need more rounded highs and smoother mids for that.