That's because most of the games made with Roblox were just amateurish and rough games, so they didn't attract the attention of adults.

Roblox Corporation created an online gaming platform and game production system called Roblox. It is a platform that contains unique user-created games of many genres encoded using the Lua ...

In effect, Roblox makes three times as much off of a creator's work than the creator themselves. Players shouldn’t look at Roblox and expect to make their fortunes in game.