I'd say it's for a few reasons. One: If the biker is using the strobe to see (bad idea, but it happens nonetheless.) the slower the strobe the longer the biker is riding without light.

Slower strobes also increase the time to be seen by other cars. Say the strobe was at a frequency of 1 Hz (a slow strobe.) If the driver looked in the direction of the biker the moment the light turned off, it'd be a full half of a second before they'd be aware of the biker, assuming the driver couldn't see the biker otherwise. A half second at 30 MPH for the car and 15 MPH for the bike means ~33 feet can be covered between the two of them before the light comes back on. That even applies for dimming lights. Below a certain luminosity the light can't really be seen anyway.

Also, dimming lights is expensive and complicated. (In terms of mass production where saving fractions of a penny off of a part makes a huge difference.)

The strobe function is SUPPOSED to be annoying. That's it's purpose... It's attention grabbing.