I know there was a custom content race designed similar to Sea Oprhans and Mermaids, By Nessy Nes/Nesoi Summer. I'm not sure really why people don't roleplay them; they'd be interesting to see.
As for Roleplaying the Tethysans Custom Content, I think it's primarily for outlying places where the custom content race wouldn't really interfere with the direct populous of w42.
Because why play them when you can play humans?!
I'd wager it's partly to do with how little-known they are, and the fact they don't come with SPARTAN lasers as standard.
I made a sea orphan. Jun Jang. He's a foreigner from the East who came to the mainland many years ago after being forced out of his Wushanko home in the Skull. He originally found the Zamorakian religion most intriguing, but after Zamorak was defeated, he immediatley stopped following the god of Chaos. Not having a strong grip on the whole mainland religion thing, he now finds Sliske's teachings and values most helpful in life, and usually goes around conning people. He wears a high necked robe to hide his gills.
Also, my character Octavius was going to be made into a sea orphan, but he was created prior to PoPs and it would be odd for that to just suddenly appear.
Also, I feel like Sea Orphans would be a really, really rare occurence, possibly becoming a larger problem for the Easterners. I assume this because they act quite hostile toward them.
I had this exact same question, by the way. That's why I went ahead and made one. :P
What do you think?