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.
If it were true that "lane-sharing is prohibited in all 50 states ", California, the UVC and most states would not have laws that refer to lane sharing. But they do. For example:
For purposes of this section, a "substandard width lane" is a lane that is too narrow for a bicycle and a vehicle to travel safely side by side within the lane.
http://dmv.ca.gov/pubs/vctop/d11/vc21202.htm
The obvious implication is that a lane that is wider than a "substandard width lane" is wide enough" for a bicycle and a vehicle to travel safely side by side within the lane." And legally.
A bicycle and a vehicle traveling side by side within the lane
is lane-sharing, by definition, given the way I defined it the way I did in the OP:
Originally Posted by Helmet Head
- Lane-sharing: Traveling fully within a lane, but alongside another vehicle in that same lane.
Now, if you want to define lane-sharing some other way, and say that is illegal, fine. But then you need to tell us how you are defining lane-sharing.