math

Sum and product are high school curriculum in many countries. Where I grew up sum symbol is curriculum in all high schools including trade schools.

Regarding readability, this case is just the definition... Problem of for loops is that they become unreadable very quickly, so quickly that most of the modern languages focused on readability discourage use of for loop exactly for readability, replaced by list comprehension or map. Once you have a real world case, sum sign become incredibile more readable. That is the reason why the meme is not how one implement a sum in real world program. The corresponding in a modern, readable language is something like

sum(x) # x is a list or generator
prod(x) 

that is the mathematical notation