You always starts with det when saying that "it is (noun)". it is a duck, it is a black bucket, it is a big star, all translates to "det er".
If there is no noun after " it is" you use den or det depending on what you are talking about.
if you say "It is (adjective)" "it" can translate to both "den" and "det" depending on what you are talking about.
Den er god
It is good (talking about a -n noun)
Det er godt
It is good (talking about a -t noun)
These structures almost always start with 'det' since they first came up 10 levels back.
If you're saying that it's just a rule in Danish, I guess you're right. Don't know what I was thinking.
I noticed that it's just the same way in German. You always say "Das ist ..." regardless of the gender of the noun.
It may help to think of "en" that indicates the gender here, not "det".
"Den" is only used in situations where the object has already been established. For example, if you and your friend are talking about a cat--"den er lille" would be grammatically correct.