Namor is supposed to be, chronologically, even older than Captain America. That means bringing him to the silver screen would create a lot of problems. To understand why, you must understand the narrative significance of Captain America being frozen in a block of ice for the better part of a century. If Cap had not been frozen, we'd have to deal with questions of how his body—whether living or dead—would have affected United States history. Was Vietnam fought with experimental super-soldiers? Was the Cold War fueled by a super-soldier arms race? We don't have to even worry about those things because narratively, Cap has been taken out of play.
In the comics, Namor has a slightly parallel history to Captain America. They both fought in World War II, and then Namor is discovered as an amnesiac homeless dude by the Fantastic Four. This emphatically would not work for the Marvel Cinematic Universe. You might think they could just say he's been an amnesiac for decades in this universe, but we have a problem here: assuming amnesiac Namor encounters water at some point, he's going to manifest super-strength and other powers. The Marvel movies and extended TV universe have established that SHIELD specializes in tracking, "indexing," and either protecting or containing these sorts of super-powered people.We also know SHIELD was active for decades. In short: it's impossible to believe Namor wouldn't have been discovered, and if he's not presented as an amnesiac, it's impossible to believe he'd stay silent through decades of nuclear testing, war, pollution, and every other threat to his kingdom.