My understanding is that your credit score is like trust, if you don't build up the trust, meaning if you don't use your card and borrow the money then how can the trust be built?
Not using your credit doesn't show that your trustworthy. Having the ability to spend 10,000 and you only spend (on credit) $1 means your trustworthy (obviously you gotta pay it up)
@Samb Who is fico to consider your card inactive?