It is not due to the time for resilvering. It is due to the rated probability of a non recoverable 1bit (or more) read error on modern drives. This probability is high enough that you have a 32% chance of it on reading 1TB. However, this is actually less of a problem on ZFS compared to hardware raid because zfs will only read actual data, not blindly every sector.
HW RAID does not read every sector blindly, there is a level of error detection there. And an errored sector in one read does not mean it errors in every read.
Now, the error detection schemes at the disk level may be insufficient. I don't know enough about how it's done on modern drives (but I suspect that every manufacturer has its own scheme).
