Fun story, which many here may know, but others won't:

The reason why these particular letters are traditionally used as counter variables, is because of FORTRAN. Which had, and still has, implicit variable typing...based on the variable name. If a variable is encountered which hasn't been explicitly declared (and in older versions there was no explicit declaration at all), the compiler infers the type:

All Variables starting with I, J, K, L, M, N are integers, all others are real.

https://fortran-lang.org/learn/quickstart/gotchas/#implicit-typing