If they're buried in the ice with the swords as their markers, Ryuma had to be buried underground, so people wouldn't steal from his grave.
You know... what if Zoro wanted to see the grave, but they told him no? Like, Kawamatsu and Hiyori never agreed to let him see it, hell, Kawamatsu called Zoro crazy for saying he fought Ryuma:
(how it could have gone)
Hiyori: "I'm sorry, Zoro-san, but... after all of the issues our country has had, Ryuma's grave has been sealed off. Only Wano royalty is allowed to go there, now. My deepest apologies, I hope that, in the future, you will be able to visit the grave, as you wished."