Yes, I can understand that you don’t appreciate Project Euler tasks. They may seem less fun that, for example, those of Advent of Code. Most of them have a strong mathematical nature and not everybody likes to deal with prime numbers and totient function. So, you have to like mathematics or, more accurately, this kind of mathematics.
As regards the usefulness of the problems, I don't think that Advent of Code or Python Challenge, the two others code-challenge I have practiced, are any more useful. If you want to learn a programming language, they are of little interest (even Python challenge). They are purely recreative and that’s fine for me. But, with Project Euler, there is something recurrent. If you don’t find a clever algorithm, you will not solve the problem. Brute force is almost never an option, except for the first hundred problems. And, if you don’t know the difference between O(n) and O(n²), for instance, you will learn it… the hard way. But I don’t see why the mathematical nature of Project Euler problems could explain why there are so few Nim users compared to users of other programming languages. Except if, for some reason, Nim users hate mathematics more than, say, Cobol users (36), Tcl users (75), Kotlin users (152) or Rust users (697). Now, as I have said, the preferred programming language of a user may not be the actual programming language he/she use. Nim is probably underrepresented, as other relatively new languages which don’t have the support of a great company.
