Why did Dungeons and Dragons come up with that Platinum is worth five times as much as Gold?

You have to remember, the worlds of D&D are not the same as earth. It is just a mechanism by which they can reduce weight and still retain the wealth characters have in pocket. When I was Dungeon Master I totally revamped the monetary system so it looked nothing like what is in the manuals.

Remember, the manuals are just guidelines, the DM can customize your world to suit your players wants and needs.

We never bothered much with encumbrance and weight unless it was obvious. We just assumed that the character was carrying around wealth in denominations that we could—usually gems.

It all depends on how nit picky you want to get in your game. Some GMs count every penny and ounce, while others (like me) sacrifice that kind of realism for story and role playing.

Since when was anything about Dungeons and Dragons intended to be an accurate reflection of real life?

Gold is pretty much the “standard”. Like a 20-dollar bill. Well, when you’re a kagillionaire, and you’ve hit the jackpot after a nice raid, that is a lot of change to carry around in a land in which banks are pretty much as corrupt as the corner cutpurse.

It’s easier to carry around platinum. Or rubies. Or pearls. Or hell, oyster shells. Whatever is rare in the land your DM has created. I think we did basilisk teeth once. There was a nest nearby, and the basilisks used to run off with the city’s children, so anyone who killed a basilisk could get the world for a basilisk tooth. Eventually trade in basilisk teeth becomes a commodity. 25 gp per tooth. Nice and light.

We didn’t much worry about encumberance, though, as long as you kept it reasonable. Obviously a Human Fighter with a STR of 18/90 could carry quite a bit more than a halfling magic user with a STR of 5.