The thing is elusive. Think like this instead:
1. Why does amp sims manufacturer come out each year with new and bettered sims, speaker IR's and so on?
2. Marshall came out with it's PLEX in the 60's. So did Fender Twins, and they left it at that. Why do you think?
If they came out with something new, it was a new model from start on.
3. Does amp sims sound like real guitar amps:
NO! And never will.
4. Does amp sims sounds like miked up guitar amps - i e recorded guitar amps in a studio:
YES! And most of them have been doing that for quite a while.
This is what you should compare to. Recording (miked up) real guitar amps. Nothing else.
It's the 4th point that we are discussing and arguing about ad naseum. It's like the derivate or something. You will get closer and closer to that guit...sorry... cigar but never reaching it totally. As we develop and get better and better,
it will be sufficient enough, for most people. Say, take Guitar Rig, Amplitube, POD Farm, or any og the big sims. If you'd buy all this gear IRL it would:
1. Cost you a fortune.
2. Weigh too much, take up too much real estate, and consume waste energy - think about how ENVIRONMENTALLY friendly, amp sims are! Especially those of CLASS A things.
3. Save your hearing. No need to crank past 120 db to get "that" tone.
There are several other things. Like, amps that aren't made anymore. Vintage styles, and combinations of ohm, sagging tubes, Power attenuators, that will shorten a real tube amp lifetime, if not even being downright hazardous to your health and even life. You can pretty much experiment with anything, and nothing will blow, or short in the virtual world. The most dangerous things that can happen is crashing your system. A restart and then you're up again. No tubes wearing out if you don't tell the sims to do that. No cables to trip over. Save the one between your guitar and soundcard/PC/MAC.
My gripe, with ALL SIM manufacturers, so far, is that they aren't picky enough. They tend to focus on different and - imho - wrong parts of the timbre. When playing through ANY sim over a dense backing track, it's very hard to make any note JUMP out when you're leaning in with pick attack. I e dynamics. Even with distortion. Not so on real amps. I have had a very cheap transistor Peavey backstage amp at the side of my studio monitors, and even on the lowest volume level, that guitar note is heard above the mastered and compressed-to-death dense music that is going on from the computer. However, the note wasn't particularly good sound wise. But you could detect it all over the noise.
Even DigiDesign ELEVEN and most of Axe FX patches suffers from this mind you. You can't EQ or "mic" your way out of these things. You have to turn up the general volume of the guitar track so it becomes annoying. Transient plug-ins won't help either.
I would love to see more manufacturers go into greater detail than before what happens to the transient initial pick attack, and what it does to especially speaker cone movement and overall tone. I find when I am playing up the plain strings on any guitar, it can sound pretty damm good sometimes, but as I go down the spun bass strings, they drown, in the attack and just turn muddy. The thing is: Not on ANY real amp, on any distortion setting, miked up or not. I mean, it should react more to all nuances of pick attack, angle and more sensitive dynamics.
What happens after that, the decaying note and sustaining note, they've sorted out a long time ago. And those things I have no complaints about as of yet. There are still some kind of intimacy lacking, though. Albeit small. And nitpicking thing by the public in large. Say Studio Devils amps, or THD2 by Overloud. No matter what amp, clean or distorted, or resolution, there's a slight "CCCCHHHH" sound or timbre present in all presets that you can't really get rid of using EQ. Same with Guitar Rig. Even if you swap between Marshall, and Fender, there's a "haze of sameness" to the sound, timbre and tone that ain't present IRL amps.
The dream is when an AMP SIM company (or reviews of it) will state:
If only a REAL AMP sounded as good as this!And even, when such a statement will come from their competitors.