Because it's redundant to prohibit things that are already prohibited. I know it goes against VC-ist dogma but, as a general rule, lane-sharing is prohibited in all 50 states (with a few, explicitly stated, exceptions, such as two bicycles in the same lane)-- even in California where the CHP has said that it doesn't care what the law is, it isn't going to enforce a ban on lane-sharing unless it feels like it.
I hate to burst your bubble, but the VC-ist dogma that says that lane sharing is "vehicular" is a gigantic load of cow chips- the "WOLs are better than bike lanes" crowd notwithstanding.
This thead has nothing to do with WOLs or bike lanes.
Anyway, from the CHP website:
Can motorcycle riders "split" lanes and ride between other vehicles?
Lane splitting by motorcycles is permissible but must be done in a safe and prudent manner.
http://www.chp.ca.gov/html/answers.html
I don't see the CHP saying it doesn't care what the law is.