I think game ideas can get copied everywhere but it happens much faster on Roblox and there is almost no recourse for the smaller developer who got copied.

The platform doesn't care when someone makes a copy, there's no way for players to push back and actually hurt the developer making the copy (unlike Steam where trying to do it will get you bombarded with negative reviews and buried by the algorithm), and superficial visual quality and marketing attracts players more than actual gameplay quality.

Depending on what is copied.

If we are talking about game mechanics, they get copied all the time, of course. If it's about superficially reproducing a game including the assets, I don't think any big players on PC or consoles steal ideas from indie/smaller developers. That is despised and likely not allowed by the platform, if not challenged in court.

(I have no idea about what happens on mobile platforms)