For the greeting of the day, in many languages, it's often expressions, with no particular reasons.
Don't translate word by word, just Imagine they use "have a beautiful day/afternoon/etc.." and you must translate it by "good afternoon" because it's the common expression in English.
For a greeting, being beautiful, good, nice, everything makes sense and is the same in the meaning (a good wish for someone), but the form only depends on the expression used in one language or one other.
Possible current English usage notwithstanding, "pretty" and "nice" are not the same thing in either language. "Pretty" describes physical appearance, while "nice" is more of a personality trait, and hezký in Czech is not translated to "nice" when applied to people. It is just something to get used to, as the course is quite consistent in this.