Because when you are writing drivers for MCUs, you are writing into arbitrary pieces of memory on arbitrary addresses specified by reference manual for you MCU. And when you will write 0xABCD into memory address 0xF120, then your UART will throw out 0xA, 0xB, 0xC, 0xD on a pin using clocks defined by register 0xF124 which is actually a divider definition from VCO connected to XTAL.

No amount of abstraction under any language will isolate you from such memory model.