Because 100 grams isn't relevant for many foods.

If I'm making a sandwich, I'm using 2 pieces of bread, not 100 grams. Having the nutrition information denominated in slices simplifies calculating how many calories/macros/etc. I am eating.

In Europe, it is really bad with snacks. You can buy a pack of individually-wrapped chocolates, and the nutrition label will helpfully tell you there is 500 billion calories in 100g. Thankfully, I am not looking to eat a quarter pound of chocolate in one sitting, but I would be interested in knowing (without getting a scale and doing the math) on what one of the chocolates contains.

Serving sizes are imperfect, and in general American food is trash nutrition-wise, but the labeling standards are quite good. My biggest complaint is the loophole allowing 0.5g and less of sugar/trans fat/etc to be reported as 0g - not anything to do with the serving size.

That is not an American thing, most packages in Europe will also include some arbitrary/common measure along side the standardised one.

The amount of something you're going to eat will vary significantly based on what it is. That said, you're going to find in the US that categories are pretty standard, for example the serving size for nuts is pretty much always 1/4 cup/28g. 100g of most nuts is way too much to be a recommended serving size

Regulatory capture. The 5 companies that make and distribute all American food want to be able to control how the nutritional content appears to people. So they can better manipulate them into eating more, or less healthy products. It's intentionally done to rob consumers of agency.

These cashews are a great example. People don't want to eat 1,000 calories in a sitting. So the company displays the nutritional information in such a way that it looks like a "serving" is only 100 calories. They do this when the salt or fat content is very high as well.

The most egregious example is that they used to do this with a single candy bar too, but a few years ago the regulatory body finally told them they couldn't do that anymore. So now the two-hand sized Snickers in a single wrapper is split into "two bars". The big-ass coke in a single bottle still gets to pretend it's 2.5 servings though.

That serving size is anything but arbitrary.

Take TicTac for example. Serving size: 1 TicTac. Which is just below 0.5g. So one serving contains less than 0.5g sugar (despite being 99.x% sugar!), which means it can be advertized as sugar free in some locations.

Or frozen pizza. Serving size: 1/3 pizza. Seriously? 1/3? Yes, because with that serving size, it keeps fat and salt ratings below the recommended daily intake.

Those serving sizes, while they are far from making sense for the consumer, are chosen very deliberate.