Why is a RAID array harder on a hard drive?

What do you mean? RAID arrays are only for storage setups - what other kind would there be?

Edit: Ahh, I see what you mean. It's still flawed though, except potentially in the context of a parity based array (RAID 5 or 6, as explained below).

The only sense in which I can imagine RAID being harder on the drives is a write-intensive RAID-5 array, since the array has to do 1 physical read and 2 writes to the hard drives for every write operation. But that implies that reads and writes somehow "use up" some of the lifespan of a hard drive, and that's a nebulous connection, IMHO.

In the sense that "hard on the drives" means "makes them do a lot more work", which translates to "they're much busier and can't sustain as many write operations per second", then yes, writing to a RAID-5 array certainly does that. To a lesser extent writing to a RAID-1 array does too because you have to write to two drives for each update. But RAID-1 writes can be done in parallel so the performance cost is much lower.