[faragon]

Short answer: shared resource synchronization.

Long answer:

You have many devices connected yo many different buses, some of those buses having shared resources allowing bus mastering and Direct Memory Access, and those devices can fail in the middle of a long operation and the OS has to handle/recover those situations.

E.g. ethernet network hardware is very simple, however, depending on the bus being used for communicating with the CPU, driver architecture would be radically different.

So, until I/O buses get unified working always in packet mode, drivers will continue being a big mess.