I suppose it has to do with the difference in input impedance
It's really more than that... I'm not saying the affordable SS devices, or the digital ones, can't offer good tones, but one thing that was common to EVERY single one I've tried, is that they are far less transparent than a good tube amp. People talk about the feel of tube amps is the big difference between SS and modeling. I've found the the difference in transparency is just as big a deal as the feel.
The transparency shows everything more clearly. Coil tap, tone or volume rolloff, different wood types, etc. etc. are more clearly defined. The SS and digital devices tend to homogenize those differences. You may still hear them, but not to the same degree. Even the higher end digital devices I've tried were somewhat guilty of this...
One of my buddies put new pickups in his Les Paul last year and the new ones were boutique, well known great pickups. He didn't like them as well after playing them for a few weeks. Knowing what he likes, and that he got the right models, I asked him what he didn't like and he said they simply didn't sound better and maybe not as good. Then he emailed me one day laughing and saying his wife was gone all day so he cranked up his tube amps, and suddenly was in love with the new pickups. When I asked, he'd only tried them the first few weeks with his Helix. He said it was "night and day better" once he turned on the tube amps. Clearly and immediately heard the difference and that it was superior. But after playing through the Helix alone the first few weeks he was convinced they were no better and hardly any different.
I can also give you another story about a friend who has a Single Cut that is awesome, and got an SE and said it didn't play as well, but sounded just as good in the store. Through a modeler. Got it home and immediately liked the 57/08's in his core model much better. Which is fine, he wanted a price point gig guitar and he'll put pickups in it. The point is, in the store with a modeler, he said they sounded almost identical.
Not trying to start a tube vs. SS or vs. Modeler debate here, but this is my opinion on this. I have not tried the newest AXE stuff.