IMHO, worthless though it is, I don't get why Diablo-likes are called ARPG's. They have lots of A and no RP. (They are Gs tho.)

Because in 1996 experience points, leveling up, character attributes, and magical loot were all closely associated with RPGs. Over the intervening roughly 30 years those mechanics have been adopted by games all over the spectrum. However the genre-name for Diablo-like games stuck due to convention.

Also Diablo being called an ARPG predates Dark Souls by 10 to 15 years.