Yup the cast is the players and the DM. Actors who enjoy having work and getting paid for it. Now you may be right it might be very hard to find 5+ CR guests who couldn't commit to the gig.
I'm not so sure though since apart from Ashley Johnson CR seems to have done so despite the case being very prolific and busy voice actors.
CR exists because Liam told a random assortment of friends "I want to play D&D for my birthday." And they did. They played a 4e one-off. Then they decided to make it a regular thing and thus they played once a month or so. They all became closer friends and two years later Felicia Day said, "Hey, how about playing once a week on my web channel?" And they laughed "Who would watch that?" And thus, believing it would only last a few months, the "committed" to play once a week in perpetuity. And it didn't stop and Liam and Sam's wives were a bit unhappy that they had to be out every Thursday. But eventually came around to it. CR also benefits from 4 of the cast making up 2 wedded couples. And those couples are all close friends with the other four.
Bringing in a handful of strangers and expecting lightning to strike again would be amazing. Also, there is much more competition these days.
Oh, and they can decide "hey, this week, we aren't taking a paycheck." and it's not really an issue. Try doing that with hired help.
4 regulars and a rotating member like Harmonquest could be even easier.
Deborah Ann Woll's Relics and Rarities had that format. But they only ran for 6 episodes. Most likely because of time commitment issues. Getting people to say yes to 6 weeks in a row is much easier than "in perpetuity".