To be fair Black Adam/Shazam are much bigger than all of those guys outside of Namor, actually the way DC has been pushing Shazam it's probably safe to say he is much, much popular than Namor. Plus most of those roles seems like they aren't even really planned in the Marvels timeline right now so they'd still have nothing to offer or work with for the Rock.
Marvel got Namor back already but there are still "complexities" that will hold it back from being made. Plus it seems they have no plans to make Namor anyways at least not at the moment.
I must be under a rock cuz if you put the rumors that there will be a Shazam movie aside I don't see much hype or notoriety for the character, even more so after losing his OG name. Comics fans is one thing but if you ask the common person about Captain Marvel you know they're gonna think it's a marvel superhero (not even thinking about Carol or Mar-Vell) and if you say Shazam I bet you they bring up Shaq before they bring up the boy who says a magic word to turn in to a superhero. I mean we are talking about movies but comics wise what exactly is DC doing? He doesn't have a solo comic and I think he's only a regular in a team book and for a while had a backup in somebody else's ongoing.
So I can't agree with Shazam being bigger than Namor or a lot of other guys at the moment. I don't see dude in any other form of media at the moment. Same goes for Black Adam. I ask somebody about him they'll think I'm bringing up the black Adam & Eve on some black Jesus tip.
At this point it's about marketing and promo to make these lesser known heroes or villains more popular the way Marvel has done with GotG. I got no faith that right now a bunch of ppl know who Shazam/Captain Marvel/Black Adam is.