And yet cyclists don't install loudspeakers so cars would notice them. A motorbike is more visible than a bicycle and can go with the flow speed so unless one drives dangerously it should be safer than cycling even without loud engine.
Cyclists are slower and plenty of cyclists will have had actual collisions or near-misses with eg car doors being suddenly opened into the bike line, cars cutting across the bike lane without looking, and so on.
Both riding a bicycle and driving a motorbike is less safe than driving a car because they are harder to notice and less protected but there should be some compromise between safety for a biker and noise pollution or a large are around the road.
As to cyclists to be slower: 1. a motorbike driver can choose to go below speed limit is the situation / road configuration makes it unsafe to go fast. 2. low speed is not always safer - on a straight road outside intersections speed of the flow (or just below) is safer.
