Lane sharing is the legal term. Lane splitting is a colloquial I avoid using because it's an inaccurate pejorative. If you lived in a country where motorcycle traffic sharing lane space was commonplace and accepted by the populace you'd understand what I'm talking about. Americans are culturally conditioned to see motorcyclists as kids, troublemakers, thugs and recreationalists. Less than 1% of commuters ride a motorcycle in the United States. Motorcycles are toys here, for weekend use. Most of the world understands the benefits accrued to a transport system when significant numbers of single occupancy vehicles are motorcycles, filtering through rather than blocking other traffic and creating more space on the roads by eliminating equivalent numbers of large vehicles from the traffic stream. How all this plays into marketing motorcycle sports in the US vs every other country on the planet is as previously stated. Adults in the US are NOT dedicated motorcyclists. Even the riders are recreational lifestyle dilettantes divided by tribalist attitudes towards one another. You can't effectively market motorcycle sports to the general public here, they don't identify with the activity. They do know and enjoy a circus, regardless of theme, so motorcycle themed circus IS marketable. Orange County Choppers, Sons of Anarchy, etc, make money on the circus, not the riding.
Many of the places that Uncle Sam let me visit while doing the Army thing had large numbers of motorcycles like you describe. Most of them were 125cc or less and not really what we'd consider motorcycles though. I can't begin to count the number of times I saw a "lane sharing" scooter rider getting cleaned up off the road with a hose and scrub brush. Let's not advocate that here.