I know that there's exceptions in the rap/hip-hop "music" world, but my overall rejection of (alot of ) it as being "not really music" has to do with the fact that songwriting has always consisted of some basic building blocks - melody,harmony,rhythm,tempo, dynamics, lyrics, arrangement and instrumentation, counterpoint, etc. - and this has been true in the history of music. It was true before people even had names for those things (that's why they had to come up with names for those things).
Of those basic elements, melody and harmony are far and away the most distinguishable. They're what makes us recognize a Beethoven symphony, or a Beatles song, or a Burt Bacharach song, or whatever it is.
Rap/hip-hop is a style based, in the vast majority, on rhythm and lyrics. Lyrics, of course, is a completely open book - you can say a literally endless amount of things with virtually no restrictions (whether they're interesting or worthwhile or not is pretty subjective, I guess, but the words themselves can be arranged into endless combinations). With rhythm there's alot less possible combinations and even far less, considering that it needs to give people a beat they can feel and snap they're fingers to or whatever.
For some people - a lot of people obviously, given the poularity of rap/hip-hop - that's gonna be enough. For others, who like to have a melody they can hum or who enjoy the expressiveness of the music itself (regardless of the message of lyrics, or whether there's lyrics at all), we're gonna have a hard time giving rap/hip/hop the same classification as a great symphony, or a great, hooky rock/pop song, or a great John Coltrane solo, or whatever, because it doesn't have the components that can invoke any feeling in us.
To use your example of Kelly Clarkson, yeah, it's over-produced, probably, ultimately, not very memorable, and somewhat disposable, but it's got all, or at least most of the components of the stuff that I and a helluva lot of other people consider "good music" and can be judged accordingly(today's rock and pop music seems to be pretty devoid of any dynamics, but that's a separate rant).
Of course there are gonna be some exceptions ( I'll let you come up with them), but most rap/hip-hop can be considered music only in the most obtuse and forgiving definition of the word.
That's where I stand, and I know I'm not alone.