If you agree that it doesn't matter whether Data's brain is positronic or biologic, how can you claim that it suddenly makes a difference if the brain is duotronic or optronic (i.e. the computer of a starship)?
Surely Data is just as simulated as Moriarty, as neither of them had a childhood. Both had a youth of sorts, though (despite being born with fully mature bodies), as they have evolved as response to social pressures.
The only real difference between Data and the EMH is that the former has a solid, immutable physical body and the latter has a forcefield-based one. Both were wanted children, but both were expected to grow into something different than what actually transpired. Moriarty was an accidental birth, but no less a lifeform resulted from that accident.
Timo Saloniemi
Because data has a brain, albeit non-biological, and it performs the same function as the human brain, and therefore results in the same phenomena of conscious experience. This is why he is more than the sum of his parts.
The ship's computer has never been portrayed as being anything more than that, just an extremely advanced super computer. The computer voice never expressed an opinion, offered unsolicitored adviced or asked any favours. It is merely a machine, much like the desktop I'm using right now, and as such, is equal to the sum of its parts.
By extension, the EMH et al are manifestations of the ship computer. The actions of the hologram are computed within the computer and projected into the physical world (in Trek lore) as a series of micro managed force fields in human form. The hologram has no brain, it is managed centrally like any other automated ship function.
The thing that made data special was not that he was a good simulated person. He had an electronic brain that was highly complex, fragile and difficult to reproduce. His conception was the initiation of the positronic net, the network of positrons builds itself and if succesful, a free thinking independent mind develops. Give this mind a body capable of receiving sensory input like eyes and eyes, and output like arms, legs and voice box etc and you've got yourself a person. Software corruptions caused by continuous running will destabilise a programme, not make it self aware.
Granting trek holograms a mind/sentience/sapience/consciousness/qualia/whatever creates implications for the rest of the trek universe and the level of human advancement that are simply not apparent. Its easier to overlook the Doctor's sentience in voyager as a writers typo, than accept that Humanity can create life on a whim and still be people, not god.