I've often wondered this or why they haven't turned your smartphone into a wireless pack at the very least. Bluetooth receivers don't break the bank by any means and from what I've heard have pretty good sound quality. I like to imagine a world where my pre amp and wireless unit are in my phone and I'm just beaming to a class D featherweight power amp on top of my cabs.
Maybe I'm just foolish though and this wouldn't work for some fundamental reason.
Latency = time delay
As fast as our technology is today, it's not instant. Everything you just mentioned is digital. It requires millions and millions of codes in binary form (which means every single piece of data is converted to "1"s and "0"s). Processing, lining up, transmitting, the signal traveling through the air, receiving, decoding, streaming, amplifying, sending through speakers, and the sound traveling through the air to your ears takes a tiny bit of time.
This is no big deal when it comes to tweaking your IEM mix. You don't even notice that when you (let's just say) boost the volume of your bass a bit it takes a tiny bit of time for that to happen. (That's why you think "Hey, if it works one way, why not the other?" But you simply don't notice the delay while tweaking your personal mix). However, when you are playing notes, in real time, trying to lock in with a drummer who is also playing in real time, every millisecond counts. IEMs run through all of this process would drive you nuts because of the tiny bit of delay involved.
Imagine if you were singing the national anthem at a Red Sox game (GO SOX!). You are standing at the Green Monster (left field wall) and the speakers broadcasting your amazing voice were behind home plate. The only way you hear yourself is through those speakers. Your brain would not be able to compensate for the slight delay. Every time you sang a note, you would hear it a tiny bit later. You just couldn't do it. Now, that kind of delay is more physics and less about technology, but the same thing would happen only a few few away because of the "process" of converting your analog bass signal to a digital signal (those "1"s and "0"s mentioned earlier) combined with transmission times would make IEMs useless because of the delay. You would spend the entire show trying to compensate for it and would be nowhere near close to accomplishing it.
If just your bass signal weren't enough, let's throw one more thing into the pot. Not only would your bass signal be going through this process, but everything else you potentially want in your mix would be going through it as well (vocals, guitar, keys, kazoo, and cowbell). Now, all of these things would be happening at ever-so-slightly different digital rates due to different versions of iPhones, different types of signals, distances from receivers, frequencies etc. So the tiny delay for each thing would be slightly different. Possibly not noticeable, but possibly a nightmare depending on circumstances.
Our current digital technology is pretty amazing. But it's not instant, even though it's kind of close. Kind of close isn't close enough in this case.
Sorry for the length but hope that helps.
