Hard drives are random-access devices and sectors are the smallest atomic unit that a drive can normally physically read and write. It doesn't read or write half a sector. When emulating a write to a 512 byte logical block with 4096 byte physical blocks on the media, it has to read the whole 4K sector, modify it with the changed 512 bytes, and rewrite the entire 4K sector.
The concept of sectors could be hidden from the interface, theoretically. You could put the whole file system into the drive (OSD), for example, or allow the host to address bytes, hiding all the read/modify/writes. But, all the common hard drive interfaces (ATA/SCSI) use blocks/sectors.