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.