There are numerous polished games on the platform, but the most of the players are younger kids on hand-me-down mobile devices, so the most popular games tend to be easy to run casual games. Here are a few examples of what you're looking for:
* Frontlines: https://www.youtube.com/watch?v=SA71d3O1ID0
* Primal Hunt: https://www.youtube.com/watch?v=X_lenB5MTTU
* Riotfall: https://www.youtube.com/watch?v=1E0kEFxBTEM
It's a hard problem, though I agree there is some low hanging fruit that Roblox seems ignorant to. I think the biggest reason for its success is an extreme lack of competition. Nearly every time someone makes a good enough (use friendly) sandbox engine, they wrap it in copyright and market it wholesale; and that makes decisiveness (polish) a high priority.