Good at.. I'm not sure how to explain why unfortunately. But other examples: a) I am good at skating.. b) They are good at synchronised swimming. c) Richard is good at snowboarding. I guess the reason for 'good at' and not 'good in' could be that 'good at' is an expression. Expressions are what they are, and are generally not grammatically correct. They just have to be learnt. Hope that helps a little.

"Good at" is a fixed phrase and there isn't exactly an explanation. Sometimes languages just have expressions that don't translate into another native language. "Good in" cannot be used to express skill ("good at cooking" -> he is skilled in the art of cooking) like this. I don't understand why Italian uses "da Luisa" to mean "at Lusia's house", but that's just how it is said.

Not wanting to disagree with Shonel, but both "good in" and "good at" can be used almost interchangeably. It depends on context, common usage by speakers in certain areas and the option you choose can have a range of nuances. Let me make an example:- "I'm not good at English" might suggest that you think of English as a school subject, rather than a language. "I'm not good in English" might suggest that you misbehave in your English class. Or even that you don't produce your best quality when speaking English. The point being that using either of the two is not incorrect. I hope that this does not confuse you even further.