They pollute more and use more energy to move a single person.
Motorcycles are worse than cars…
How exactly does a motorcycle that gets 60 mpg (3.92 l/100 km) take more energy to move a single person that a car that gets 25 mpg (9.4 l/100 km)? Notice that almost nobody carpools in America, which is the subject of this post.
Also note that almost all motorcycles sold worldwide comply with Euro 5 emissions standards.
Emissions systems. Do you realize just how toxic something without a catalytic converter is? Let alone one that doesn’t work for shit like the types installed on bikes.
Also a car is probably FAR more efficient at burning fuel than a bike. The efficient burn lets out even less toxic gasses despite using 3x the fuel.
Take the totally emissions free lawn mower market and scale down emissions a bit since bikes do have some emissions equipment and electronic fuel injection. But they really don’t have much, and they’re very often just removed/bypassed.
https://psci.princeton.edu/tips/2020/5/11/law-maintenance-and-climate-change
Why the fuck would someone compare a shitass lawn mower to performance road hardware?
On emissions, read and learn: https://www.motorcycle.com/features/what-you-need-to-know-about-euro-5-emission-standards-for-motorcycles.html
Thermodynamically, your argument doesn’t make sense either as you are still injecting 3x or more waste heat into the atmosphere, nevermind the CO2 (which is a crazy thing to ignore in this discussion).
The last part of your statement is entirely an opinion based in ignorance, people are generally not interested in dramatically altering the reliability of their machines, and manufacturers keep the cat separate from the muffler for bikes where the frame has room. Have you done your own moto maintenance? What bikes have you got?