Question

Why is it not fair to assume that, just because a person is good at one thing, they are good at everything?

Answer 1

Because you are then holding up an expectation of them to be good at everything. then they feel like they actually have to be good at it, because maybe they don’t wanna disappoint you or be embarrassed because they aren’t as good as you held them up to be.
after you have expected them to be good at everything, and they’ve failed, they probably feel like a failure, like they could have done better. you’d basically be signing them up for failure and embarrassment if you expected them to be good at everything after you realized they can do one thing good