Why is .toString range limited to 36?

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Bad_radix

Why is it limited to 36? A radix that is larger than 10 uses alphabetical characters as digits. Therefore, the radix can not be larger than 36 as the Latin alphabet has 26 characters only.

what's more, with radix 18 you don't get hexadecimal values. hexadecimal is base 16