Another reason is purely practical. If all the investors have the same leverage, we don't need to calculate each investor's bankroll size after every bet. All we need to store is what percentage of the bankroll is theirs. If you have 10% of the bankroll before someone bets, you still have 10% of the bankroll after they bet. All I need to update is the size of the bankroll. If you are using 2x leverage and I'm using 3x, my share of the bankroll will increase and yours will decrease each time a player loses a bet. That adds additional complexity that I didn't want to deal with.

This likely means that the "real leverage" player does worse than the fixed offsite investor in the much more common case where the player wins and loses more equally.