many of them can do it much better than aretha. Its just that at the time she was a stand out lady for the talent pool available in her generation .People say similar things about all the greats of their day like Michael jordon for example but there are better players today or Michael Jackson but the talent pool is deeper now and its harder to stand out.
AI is a TV show and they have to have all the ingredients to make the show successful. Unfortunately that also means that the best singers wont necessarily get picked if they are too simmilar to other singers in the show.
The best singer there Aaron Marcellus/Sanders (IMO) did not even get any TV air time .
check him out
http://www.youtube.com/watch?v=1Cdt3CGHMpUIt wont be to everyones tastes but it touches me.