No one is talking about magic. The human brain is not magic, neither is that of chimpanzees, rats, dolphins or gorillas. Intelligence is a purely physical phenomenon, which means it can be emulated by computers. Natural brains are also a product of evolution, which means (1) the development happens very slowly, (2) development is directed only towards evolutionary success and (3) there is no flexibility in how the thinking organs are constructed. Computer intelligence does not in principle have these limitations. It would be terribly anthropocentric to believe that humans are the most sophisticated intelligent entity that can exist in the physical world - after all, we are as far as we know the first such entity to emerge, so from our perspective the evolution of intelligence has now stopped.
That's the feasability argument. The risk argument is that the consequences of an independent, runaway intelligent entity significantly more capable than humans would have such devastating consequences for humanity's future that even a small risk merits a significant effort to map out the territory. Respected scientists have said "it is impossible" to hundreds of things that proved to be quite simple, so this is not an argument. Even if you don't buy the previous part of my reasoning, there is still risk here. In principle, there are any number of things that could preclude advanced AI in the near future even if the above reasoning is correct (too difficult, requires too much computing power, uses different computational techniques), but seeing as we don't know the unknowns here, taking the cautious route is the correct thing to do. This has been a scientific principle for decades, no reason to drop now.
Do AI researchers have any arguments opposing this that don't amount to "the AI we have created up until now is not very good"?