Actually they can be. And of course the biggee is just the ability to go into a studio and do 100 takes if that's what it takes, mix, match, sample, and if a singer is flat 9 times out of 10 in any one part of the song, you take a bit here, a bit there, paste them together, adn voila! release the single. Then you lip synch the music onstage while they pipe in the hyper produced version over the speakers and there is no way to tell a singer who can hit every note on the first try from one who hits them 1 in 100.
Good tests BTW are singers who step forward to sing national anthems or other live type performances where its not their studio stuff. Christina Aguilera for instance is a pop princess, but she can actually sing removed from that. Ditto Mariah Carey. Not so much many MNAY of the others.
And as an aside, a performer like Britney is a name and an asset. Its not about talent after the whole thing is established. Its about brand name recognition. Studios have pumped millions into her image and name, and they can get millions out of her with a fraction of the effort it would take to pump up and create a new image/name recognition for somebody off the grid. Does not make her untalented necessarily, but it means that there is absolutely not any sort of fair competition between her talent and the unknown singer's talent.