The difference is that Roblox has a thousand "attention stealers" which have enough gameplay and multiplayer fun to keep an eight year old entertained for a long time. Fortnite is just the same recycled concept over and over, with an interface that is difficult for a child on a console. There are a number of Roblox games that are genuinely well-designed and fun, don't let the graphics fool you.

(Also, eight year olds don't have $3 in Robux unless someone buys it for them, so blame the parents as well)