'Rock' is an umbrella term referring to an area of music containing many sub-genres that has been evolving for nearly 70 years now. Even at its birth, it evolved from multiple genres such as Country, Blues, and Boogie-Woogie piano music. As time has moved on it has taken in more and more influcences such as folk, classical, jazz... and ultimately, every other genre out there. Accordingly, it has incorporated harmonic pallettes from all those styles - and in some ways, found ways to fuse them together.
most of the scales in rock are either major or minor
First, a question - where does that assertion come from? is it your impression or does it come from a statistical analysis of a corpus of work?
Doubtless there is a lot of rock music that is based on major and minor scales. There's also an awful lot that is based around the blues scale - I bet if you asked 10 guitarists to play a 'rock riff', more than half would play something based around a blues scale. There are a lot of modal influences in rock too. (I've sometimes thought that there seems to be a kind of 'Rock scale' or 'rock key' that incorporates aspects of Major, blues, minor, and perhaps the Mixolydian mode.)
As an illustration of something that exhibits a kind of bluesy rock tonality, Have a listen to Joe's Garage. The first part is fairly straightforward major - then pay attention to what happens at about 4 minutes, when they're making it sound more 'rocky'. They're certainly not sticking to straightforward major - or minor - at that point.
blues scales are created from adding chromatic notes to a pentatonic scale.
It's not as simple as that. The archetypal blues sound is all about bending notes through ranges of pitch (which you can hear a little of in the Joe's Garage vocal). There are simplified versions of the blues scale which could be described in the way you have, but that wouldn't qualify as a full description of classic blues tonality.
Isn't rock more similar to say classical or pop music
Maybe - but only in that rock, pop, and classical are all huge, sprawling families of genres that have adopted many different influences over the years, and include examples using many different types of scale.