But they are not.

From what I understand, you can't go to a shop and be presented with an array of 100 different types of seatbelts which you can quickly and easily install in your car as your whim changes.

But you can do that with bicycle lights. You can pick whatever type of bicycle light suits you ... or not.

And people have different needs when it comes to bicycle lights. Some people never ride in the dark or in foggy/gloomy conditions. Some people might want a light just in case they end up in those conditions, but the light only needs to last for a couple hours. Some people cycle all night long and want a light that lasts all night long. Some people just want to be seen on the bicycle and don't need a light to see by.

And when it comes to bells, some people like to express their personality with their bells and so they'll select one shaped like an animal or something flashy and pink ... others just like a small plain silver bell.