Why is parallel I/O slower?

Question

Iron

I found out that it is undesirable to have more than 1 thread for Disk i/o , not network, namely disk. By the way, here's one example: Why is parallel Promise.all() an order of magnitude slower than sequential awaits? .
Googled - and did not find a detailed answer.
Although there is some speculation as to why this works this way, I would like to hear an authoritative opinion or see some kind of link.