Super Mario Bros.: Why all the floating money?

[Gilbert]

There can be a lot of reasons. Some that I can think of (not necessarily means these are what the developers thought):

1. Coins can be animated in a more interesting way to give more life to a game (like rotating in some of the Mario games). How do you animate money sacks? Gold bars? (Maybe, flashy with glitters, but the result may not be good with the graphics capabilities of old hardware). Note that in the original Mario Bros I think (not sure, need to double check) the coins may bounce on a platform, which could look out of place if you use sacks and gold bars. (For bouncy coins, check also Fantasy Zone. The sequel added banknotes, but I think it didn't work as well, and it took me a long time to realise they are banknotes).
2. Coins are easily recognisable, especially on primitive video hardware. To convince someone a sack contains money, we usually draw a dollar (or pound, yen, whatever currency you're using) sign on it, which may not be easy for a small sprite on hardware with limited resolution and colours. For gold bars see the glittering comments above.
3. It is more natural to connect coins with interesting and funny metallic sounds, e.g. the sound made by collecting coins in SMB. I'm not joking, but some analyses did mention the satisfactory feeling gained by hearing the sounds of collecting coins repeatedly was one of the reasons for the success of the game. That Nintendo later even used it in their logo should tell you something.

I don't know why anyone would even consider credit/debit cards etc., especially for kid-friendly games, this is not relevant to most kids' everyday life (yet), and the sprites may be even less recognisable than banknotes in an action game . Actually, debit cards aren't even a thing here.

Edited to fix some of the typos since I was in a hurry and used a list for easier reading.
_{Also spammed it with added links because yeah, I am a bot!}

[Pokun]

Yeah coins in the Mario series made their first appearance in Mario Bros if I'm not mistaken. They bounce in whenever you defeat an enemy, so they make the most sense in this game as money bags or gold bars wouldn't roll. I always imagined it is Mario's and Luigi's plumbering wages literally rolling in through the pipes.

Bonus items in games are often based on primal needs like food, gold, jewelery and pretty girls (as the gaming world is quite dominated by males) to make them more appealing. Bubble Bobble, Wonder Boy and Joe & Mac games are all full of fruit and other food which is also probably part of their success much like the coins in Mario aided by the satisfactory pling, and they all contain pretty girls to be saved. Even Pacman has a fruit in each level as a bonus to supplement the featureless dots.

Fruit in the arcades was common long before digital video games, as it's a staple symbol of things like slot-machines. Apparently originating in early slot-machines giving out chewing gum as prices with the fruit matching the taste of it (the well-known BAR symbol was also supposedly originally a picture of a pack of chewing gum). But the delicious looking fruit was perfect to get people attracted to other games as well, whether they give out prices or not.
For me seeing fruit in a classic game is quite nostalgic.


Speaking of credit cards, Paluthen no Kagami / Kid Icarus actually has a credit card, and works like one too (you have to pay off the debt). It adds to the funky humor in this game.

[tepples]

Williams's Smash TV (1990) has currency and prize boxes on the floor worth points, as well as keys to enter the Pleasure Dome. (In the early revisions of the arcade version, the Pleasure Dome was still under construction. A patch unlocked it.)

Virgin's Cool Spot (1993) doesn't use sprites. It uses 7 UPs. It's a platformer where coins function as keys increasing the COOL % meter. You need to collect the coins and spend them on passage to the next stage.

In "The Great Cave Offensive" in Nintendo's Kirby Super Star (1996), a secondary objective between/after defeating the four bosses of the main loop is finding all 60 treasure chests, which contain prizes of various values from 10 G to 990000 G. All chests are resting on something, as the player needs to put Kirby in front of them and press Up to open the chest and claim the contents. The first two have a "Gold Medal" and a "Gold Coin".