It does not matter if the money is virtual or not. It matters whether it is regarded as valuable. If a large group of people trust a certain currency to hold a certain value it becomes relevant to those people. They might invest other forms of money or energy in to that currency. If something happens with that money or currency people do panic, because something they have invested in is under attack.

And even with Monopoly, during that game the virtual monopoly dollars hold value. Just during the timespan of the game. They give you certain privileges during that game.

When you pay rent with virtual money suddenly it starts getting more serious.

All money is virtual.