A lot of jazz is heavily improvised, and I know that many musicians typically rely on fake books and their progressions to build improvisation on top of. My question is, why aren't there fake books for blues? It would seem to be a pretty great tool to have for an aspiring blues improviser. Is it because most blues music sticks to much simpler progressions than jazz?
In addition to traditional Blues being simpler and less diverse than Jazz, historically Blues is played in smaller groups, or even solo, and more often by musicians who learn and play by ear. In that sense (and in others) it feels a lot closer to Rock 'n Roll than to Jazz. I would say Blues is more of an oral tradition, and Jazz is more composed in a similar fashion to Classical music.
As others have pointed out, if you can play a pentatonic minor scale in every key (you might survive quite well just knowing five or so keys), and you pick up or invent a few other good licks, you can improvise over most Blues progressions.
Furthermore, from a certain point of view, there are fake books for Blues, at least a little bit. There is an excellent transcription of one version each of every song Robert Johnson recorded. A few quick web searches confirm that there are song books for all of the most famous names in Blues I could think of in thirty seconds or so. So if you want to get super cerebral about your Blues, the material is out there.
Blues songs, by definition, basically, follow the blues progression as found in 12 bar blues. Whilst there are lots of variants of the 12 bar blues, they are basically the same format. The fake (or real) book would be rather thin due to this. Yes, there are other songs which come under the auspices of 'blues', but in comparison to jazz standards, they are few and far between. If one can find one's way round basic blues, there's probably no need to refer to a compendium of blues songs.