@Liquidxlax
It's because at the start of each part the chapter count and volume count both reset to zero.
Part 1 and 2 could feasibly be linked together but given that Part 3 needs to be separated and the above point it'd be really weird and messy to not separate them. The reason Part 3 needs to be separated is that it's by a different artist to Part 1/2 and both Part 2 and Part 3 are currently publishing at the same time.
But yeah each part is a sequel to the previous part and they're all the same story but different arcs, however as I've said the manga versions of Part 2 and Part 3 are both being released at the same time so even if all the manga was scanlated people couldn't just finish Part 2 and move onto Part 3 because Part 2 isn't even done yet. A similar thing happened with the Re:zero and Mahouka manga.