why is it that?

Depends on the state - but basically it is any motorcycle not specifically HD - I have pipes on a Honda Shadow - but am very very careful when going through town - no louder than a tractor if I take it easy- - some people just have to draw attention to themselves I guess.

Call the police and local politicians - and complain I am sure there are already noise ordinance on the books that need to be enforced.

depends on how short the police budget is- a rented decibel meter can be used to do sound checks during inspections and $100.00 plus court cost tickets issued, gets a few thousand in fines in couple months. As for the Harleys with open pipes? how old are they? Vehicle equipment laws based on manufacture time can have that little noisy loophole- my BSA chopper as a 1966 wasn't require to have a muffler on the open pipes- wasn't required until 1968. California Choppers with open pipes if bike was before about mid 1960s have same loophole- one simple washer baffle does the legally required job for the old ones and newer ones at times get pulled over and ticketed with a fine if police have orders to inspect and get the area quiet.

There probably are laws on the books in your state or city regarding allowable noise.

Be aware that plenty of police officers are motorcycle riders and their weekend bike is a Harley.

(I'm not going to name the city where the police force got Harley police motorcycles at a time when those cost twice as much as the Kawasakis.)

Complain to your city council, photo document illegal motorcycles without matching exhausts: include license plate photo and unmatching exhaust photo , ask that repair citations be issued for any motorcycle without matching identification. Once a motorcycle operator runs through a repair order it can be confiscated and impounded. Once impounded Federal violations can be observed and the motorcyclist can be forced to pay, tow, storage charges, and pay for the violating motorcycle's destruction costs or face Federal charges.

My suggestion would be to take your concerns to the local police dept. In my state there are safety inspections needed to drive on the roads and it clearly states in the inspection book that the exhaust system can not be modified. And every motorcycle comes new with a muffler so if there not there it is a violation.

Actually you do not live in a part of town where there are a lot of them, they specifically travel there to ride past your house, some of them from over 100 miles away.

Depending on the area (city, state, county) there are probably laws about noise. Motorcycles are typically ignored when it comes to noise pollution due to the fact that the sounds increases "visibility" for these smaller vehicles.

This will answer your question:

https://www.youtube.com/watch?v=xGyKBFCd_u4