Why is that?
Putting more money into system doesn't add any additional goods. You only get the benefit if you are among the first to get the money because you will be able to bid up the price putting the person without the extra money at a price disadvantage --or-- you get the product(s) first leaving others to go without.
The fear is in extreme cases, this can lead to inflation and/or empty shelves.
Maybe with something like donuts, the baker has plenty of dough and will hire more and create more to meet the demand. Or maybe you have no control over your raw materials (like the price of OIL or some complex refining process) and can't make any more gasoline no matter how much money your customers have.
I don't think goods are in short supply.
> you get the product(s) first leaving others to go without
Luckily the person able to solve that problem—the producer of those goods—just got a whole bunch of money by selling the goods they had produced so far.
I can understand that printing more money leads to inflation. I'm having a harder time understanding why taking money from one place and putting it in another does the same thing.