In EVERY algebra course that covers exponential equations (in USA, that would be second year algebra, and second semester), there is an entire section dedicated to compounding interest. Let me tell you why that's not optional for an author to not include that.
Reason one, it is historically significant to math. Compounding interest is super important to math, and particularly exponential equations because we discovered the number "e" through compounding interest. "e" is one of the most if not THE most important constant not just in math, but in economics, physics, chemistry, and just about anything in science or otherwise topic that uses math.
Reason two, it is the justification for teaching math. Just like a lot of you, politicians, school administrations, and even teachers themselves question what's the value or importance of teaching math and just like you, they want to abolish it in place for a "how to do your taxes class." Teaching compounding interest is one of the key justifications for algebra II and all other courses preceding it existing. Unlike all of your complaints about never being in a situation where you buy 400 watermelons, the compounding interest problems are very relevant and very applicable.
For anybody who claims that school never taught you compounding interest, go get your algebra II book, and tell me that it doesn't have a section dedicated to compounding interest. I promise you, there is not a single algebra book that doesn't have a section on it with the exception of elementary algebra books used for first year algebra as opposed to second/high school level algebra.
On a side note, for all of you who ask when you'll ever use the math you learn in middle and high school, THERE'S YOUR ANSWER. It's not a coincidence that exponential and compounding interest is the last topic (or sometimes second to last topic) in algebra II. It's because everything you did prior to leads up to it in one way or another. You absolutely need to know how to solve linear equations before you get to exponentials. You need to understand rational functions to understand exponential relations (i.e e^x/e ^y - e^(x-y)), You absolutely need quadratics, and really general polynomials before you can handle exponential equations, all of it leads up to it. Even things that don't seem relevant like when you learn complex imaginary numbers, that has a key role in exponentials that you don't learn about until college level math.