The issue isn't who's on the list or who's excluded, it's that the list is too short.
It's kind of like making a list of best basketball players without making considerations for positions, eras (and therefore competition), teammates, etc. That said, drummers and bassists laydown the foundation, keyboard players fill in the harmony (usually more expansively than guitarists), guitarists wade between harmonic development and melodic extrapolation, while horn players (in the session world) go between the aforementioned extrapolation (generally at a higher harmonic level than guitarists) and awesomely unique filling out of the harmony (i.e.: horn sections).
Yes massive generalizations on my part, but I'm basing this on their role in popular music recording, arranging, creating, etc. in the studio. In more specific genres (e.g.: jazz, country, funk, etc.) and live it's a similar, but different animal. There are a lot of folks that didn't and couldn't cross over to other or just didn't want to for myriad reasons.
I used to want to be a sideman. Depending on the artist, you might only get to play amazing music with an amazing artist and other stellar musicians, you might get to be a part of the creative process. Ideally, however, it would have been being in a band and filling the role of someone like Mike Campbell. He's another great studio guy, but focused his work on Petty with wonderful guest appearances on some great albums.
Lord I'm a borin' ramblin' man...