It is firstly because we can now.
Bulb photography can indeed shoot exposures of minutes to several hours, depending on the camera. Using a film camera, astrophotography is done with very long exposures and those cameras have no time limit since they do not need power to operate.
A digital camera can be used in the same way but most mirrorless limit bulb exposures to 30 minutes or an hour, making it impossible to shoot a longer exposure, so there is no choice.
Most DSLRs though can take exposures of over an hour, so they can make the very long exposure to get the final image in one shot. However, using multiple shots can be advantageous. Most importantly, the maximum brightness of each pixel is virtallu expanded. In one shot, once a photosite is saturated, it will be over exposed. With multiple shots, it is possible the saturation will not occur, giving software more data precision to work with. Grosely over-simplifying, each doubling of shots give you an extra bit of precision and dynamic-range roughly. So take just 4 shots, gets you 2 more stops of dynamic-range compared to a single exposure.
Multiple exposures have the effect of averaging noise. This will give you cleaner images to work with but each image can have software noise-reduction applied which is more effective than processing a longer much noisier exposure.
It is rather hard to meter for a very long exposure but if you have many images, you have flexibility to not use the whole stack or control software blending to avoid over-exposure, particularly if you have included some foreground in your composition.
The two disadvantages of doing multi-image capture are minor. One is that it is more work later since the stack of image must be transferred and processed by computer, instead of a single image. The other is that there may be tiny gaps if you are doing star-trails while the camera takes time between shots (make sure to turn off Long Shutter Noise Reduction if you do nou want huge gaps) which can depend on a number of things and the particular camera.