As
@FastST said, the wind blast will not clear a bike's windshield, but it WILL clear the face shield on your helmet. I suspect the difference is the curvature.
Not only are visors per-treated with water repellent, they also get cleaned way more frequently...
And when selecting a height for my windscreen, I always ensure to still have the right amount of airflow remaining on my helmet and visor to keep the vents functional and get droplets blow off the visor... (having to reach up there every five to ten seconds to wipe off the mist to maintain vision can get painstaking and hinders operating the motorcycle...)
I use
Nikwax Visor Proof on my Shoei lids since years...
Back to the windscreen:
- Bike Spirits spray cleaner and polish; great stuff, removes bug-splash and tar-spots with ease, seals minor scratches, keeps repelling water for some time (~2-3 weeks, depending on the solvents/substances from the road spray... spray from muddy roadworks or winter brine will kill any repelling abilities almost instantly, but ease cleaning afterwards...)
- Meguiar's G17110 Headlight Protectant; cleans not as great and easy as the Spray Cleaner, but claims resistance against environmental/UV influences; repellent properties as above...
- aviation grade pedant (EU sourced) to the already mentioned Plexus; again not as good in cleaning as the first, but really good in refreshing "older" plexiglass (i.e. instrument glass), claiming significant UV protection, repellent abilities as above... (an aircraft is rarely caught in road spray though...)