I/O-bound tasks typically do not parallelize well because disks are a shared resource.  When you launch a process with 64 processes on a 64 core machine, you get access to 64 times the compute.  If the same process is creating 64 write requests to a hard drive, you typically still see a little speedup due to "pipelining", but you quickly saturate the disk's ability to fulfill the needs of the 64 cores.  There are similar issues with memory bandwidth and network.