Blizzard has a knack for getting undeserved credit. Warcraft over Dune 2, World of Warcraft over Ultima Online...
To be fair, as much as Blizzard has been plagued by a lack of originality all along (excluding the genre-defining Diablo which ironically was acquired from another company), they've always been able to get more out of other developers' concepts than the creators themselves. But yes, a lot of the credit is undeserved. Both the gameplay and content of their flagship franchises are rip-offs that only slowly evolved into their own thing.
Agree up to the point that you say its undeserved credit. Are they original? Hardly. But neither was Star Wars or any fantasy setting after Lord of the Ring (which was lifted from folk tales). There is zero shame in taking something and molding it to your own design. What Blizzard does best is take an idea and develop a story to reach a ton of people, casual and hardcore alike. I mean, how many people have died playing WoW and Starcraft yet you can sit down and play through having never played a game in your life. Thats what they do better than anyone else and they deserve a ton of credit for it, its not an easy thing to appeal to the masses.