Because if they say it kills 100% and later it’s found out that it doesn’t kill 1 bug (out of countless billions), they will be sued for false advertising. And that’s very costly.
Then there’s marketing. Two identical products sit side by side on the shelf. One claims it kills 95% of all germs, while the other claims it kills 99.9% of all germs. Which product do you think customers will buy? “Extra strength”, “heavy duty”, “free”, “bonus”, “double” … these are just a few of the words that advertisers put on their products to entice customers to purchase their product over their competitors. Color, smell, shape, wording, font size, product placement in a store … These are but a few of the numerous tactics advertisers use to get you to buy a certain product (and they work!).