The same reason every blockbuster movie is made, Money. CBMs are at their hottest point, previously they were rising but a Namor movie wouldve been too risky as they'd have to develop a way to shoot the underwater scenes.
They'll use Namor and his supporting characters and villians. It's the exact same thing as almost all solo comic book movies and movie-making throughout history.
Blade Trilogy, Spider-Man 1-5, Iron Man, Hulk, Incredible Hulk, Daredevil, Elektra, The Wolverine, Captain America The First Avenger (sans Fury), Thor, Thor the Dark World, Iron Man 3 (sans the pointless rebranding of War Machine), Superman, Batman, Kick Ass, Dredd, Hellboy and on and on....
The only thing a Connected Universe does is have them team-up every so often and include references and so on to other characters. It's not a requirement for making movies.
Personally I think Namor would work better as a supporting-main character in either the Fantastic Four Franchise or the X-Men franchise as those are who he mostly associates with but a Solo movie has every opportunity to be great.
As for your last point, movie-goers and Fans will be the one to see it. They wont dismiss a movie just because it's not part of the universe fad, they'll see it if they like the look of it.