Lets say you need to find the probability of something happening 10% of the time to 40% of the time, you need to perform definite integration of the curve ( lets say normal curve ) from 0.1 to 0.4 on the x axis multiplied by the normal curve function. This is one of the easiest examples I could remember from my undergrad. We could solve these problems with ease at undergraduate level because we grinded hard during our high school. And also these type of problems were just a subset of the huge variety of problems presented during our undergraduate. But lets say they started teaching calculus only during Undergrad it would have become a tremendous task just to first learn about calculus then start with applying it on other subjects. I am all in for teaching calculus during high school.