The simple fact that Namor is bi-racial, and that's how Everett created him. As bi-racial person, I can tell you first hand, you are considered neither of the races of your parents. You are always seen as half of one at best. Namor's features, and even more importantly, his ears, guaranteed that he'd never be considered a white person, he was always going to be Other. Folks have pointed to Everett's original choice of red hair for Namor as indicating he was white. But at that time, red hair wasn't considered a good thing -- consider the phrase "beaten like a red haired step child." It was another signifier of someone that was considered less than acceptable.
Also, in that era, having white skin didn't make you 'white' in America. Not only did we have the "one drop" rule concerning brown people, but the Irish, the Italians, the Jews, etc. were all considered either non-white or less than acceptable.
Namor was one of those SF aliens, who have white skin so the white readership can identify with them enough to get the "message" the writer might be delivering concerning our own prejudices. People who aren't "white," OTOH, recognize the coding, and also relate to the character.
Christopher Priest recognized this in his interview on Newsarama a while back.
https://www.newsarama.com/25496-prie...k-panther.html
Obviously, some people disagree on how Namor looks. John Bryne said he drew Namor as "Chinese looking," because that's how Kirby drew him, so clearly, he, and he thought Kirby, saw Namor differently.
I like Shang Chi's comics, but he's a stereotype, and for many years, one that kind of made you wince. You don't have to know kung fu, wear Buddhist prayer beads, speak an Asian language, or even have an epicanthal fold to be considered Asian. Some of the whitest skinned people I know are Asian.
And the bolded is part is sort of what I'm saying. Mr. Spock, like Namor, is a non-white character. Mr. Spock was bi-racial and from a foreign race, just like Namor. They both have white skin, but they are clearly both Other. Mr. Spock could have easily have played by an Asian. Instead, he was played by a Jewish actor, a non-white actor.
I'm not saying Namor IS Asian, i.e. Princess Fen and the Atlanteans are Asian. I mean, I guess you could come up with some sort of pre-historic plotline where the Atlanteans were actually the descendants of people who came the area that is now the continent of Asia, who ended up colonizing Atlantis. But that Namor is non-white, and looks enough Asian, that an Asian could play him. That doesn't mean the movie or character would have to lose the Greco Roman stuff you like, anymore than MCU Asgard adopted an African motif, because Heimdall and Valkyrie were played by black people.