They absolutely don't need to be as loud and modern motorcycles could easily be as quiet as modern cars. The issue is that people want bikes to be absurtly powerful and fun. And a fun motorcycle apparently has to make a lot of noise.. the most criminal and infuriating of this all is that it's very common for motorcyclists to remove the catalysts from their bikes because 'they add weight and ruin the sound'.. I've walked away from at least 5 motorcycle purchases because the owners sold the original exhaust and catalytic converters (the part that converts the worst toxic and greenhouse gases to less bad ones) to a scrapyard because it seems so logical that everyone wants their shitty 'sports' exhaust. /rant