Only banks, hedge funds and corporates can trade on these exchanges because the minimum contract values are so high.
Most importantly, gold is physically difficult to trade because gold is:
-Not as easy to transport as BTC
-Not at all uniform (one piece of gold is not always equal to another piece of gold, even if they are the same weight).
Price manipulation may still happen (and arguably happens already) but it will be alongside a large amount of legitimate trading as well.