As one Creationist put it,
"The stubborn fruit fly has endured every genetic indignity possible, but so far not one has ever produced anything except another fruit fly." -- Page 163
"No new species has ever been produced, while the mutants have invariably been deformed or in some way are less than normal." -- Page 162
In The Minds Of Men, Ian Taylor
Yes, they are still fruit flies. But that's reasonable. There is an obvious thing to do if you wanted to create a non-fruit-fly, and all the researchers have (for excellent reasons) done the exact opposite.
The point of these experiments was to learn how genetics works. So, each experiment starts with the standard pedigreed stock of fruit flies. Those Drosophila are guaranteed to be normal, and the same as the ones used by everyone else. The virtue is that the researcher can look up all the things that are known about the pedigreed stock. This makes it enormously easier to detect any change that an experiment has caused. It's practical.
Darwin said that evolution was due to accumulated change. If scientists had been trying to accumulate their changes, we would by now have fruit flies that were 2500 generations away from normality. But since each experiment starts anew, the mutants are rarely more than 5 or 10 generations away from normality. And when an experiment is over, its mutants are usually killed.
And in fact, someone recently compared lab fruit flies to wild ones, and discovered that the wild ones have changed.
Just because they're still fruit flies, does not mean that they are still the same species. In fact, lots of researchers have created two populations of fruit flies that each breed among themselves, but that refuse to breed with with the other type. By definition, those two populations are different species.