The hard part there isn't a browser vs native thing (native apps also generally ship with all of the assets in a front-loaded client) but abstracting your rendering pipeline to handle streaming or incremental LoD over the network.

It's more of challenge of taking the time to add that level of polish.

Kind of like the rare native game, like League of Legends, that will let you play before the entire client is finished downloading. Being a native app didn't give Riot Games that for free, they had to specifically build it for their game. Even in the native game market, it's AAA-level polish for a small fraction of games.