I've always been curious, and maybe someone can enlighten me. Why are these 8 episodes so difficult to get put on the series. Was it a copyright issue? Maybe something if put out today would be a touchy subject? I'm not sure, but this has been on my mind ever since the first "complete" series box set.
If you go to the "Scooby-Doo, Where Are You!: The Complete Third Season (1978)" page, Michael E. Pajak explains all that. Basically it's not that they were difficult to put on the series, it's just that they weren't part of the original '60s series, they were produced as part of a set for the late '70s anthology series. The key aspect is that they portray the team before the addition of Scrappy Doo etc.
Amazon does have a streaming package that includes the missing episodes. I believe it's called the Scooby Doo Show. Starts with High Rise Hair Raiser and Ends with the Creepy Cruise and Ozark Witch. If you buy the package, you also get all the ones for the SD Dynamutt hour as well.