You can change category to apple, for example and topic to pear, or any other word you like.
But you might also want to ask why few forum owners do that. One reason is certainly that you need to figure out how to do it (work) and then do it (more work, though it’s easy) and then you need to keep an eye on these settings when you install plugins or update discourse to make sure that the wrong terminology doesn’t creep back into your forum (probably the biggest piece of work in the long term).
Another reason is that topic and category are not rsndom terms but make sense. You’ll find a number of topics on that if you search here on the forum for the various alternative terms you might want to use.