But they are around. Especially from reputable refurbishes that are in the business of acquiring them and/or parts and putting them together. .When you buy from random people on the internet, they might easily mis represent an older flashed one, or they might not even know. The reputable refurbishes are straight up about it.
Why does it matter? It may or may not ever matter. For me its peace of mind. They are different computers. Even though yours is flashed in order to pretend to be newer then it is, it may fool OSX into working, etc.. but technically its not the same and so far so good, but for example, you may not be able to get Catalina to work on it, while the 5,1 probably will. Just hypothesizing. I know when I upgraded the CPU on mine, what I found was that I needed to buy a different kit for mine then what you would need to buy. They are different computers. Yours is just tricking OSX into letting it run on it with a faster CPU...and that works for Mojave. We shall see about Catalina.
Some people might even pay a little more for a 2012 then a 2010 even though those truly are exactly the same computer. But just knowing that one is officially 2012 vs the other 2010, gives some people a better peace of mind somehow, therefore, its worth a little more.
But the flashed 4,1's, I'm really happy that those people have been able to keep them going, but still...it is what it is...its basically a hackintosh now. And its not quite the same as the 2010-12...I can't remember the exact spec differences. I seem to recall there are some differences with the PCI buss too.. Starting with Catlina my 5,1 will be a hackintosh too... albeit a newer generation then the 4,1, flashed or not.
In any case, I personally avoided the flashed ones when I shopped for mine that was a decision I made and chose to spend more. I think a $1000 flashed one would certain be an entirely interesting stop gap measure. I mean hell, use it for a year or two and you got your money's worth. But I prefer the 2012 variety at this point.