I am a very clear beginner in Esperanto, just here for conlanging, but why don't nouns after estas form the accusative?
For example:
Why is this?
The reason is that in a statement like "ĝi estas kato", the "kato" is still referring to the same thing, which is the subject of the sentence. The accusative case is needed when you're using a verb that talks about two different things.
The accusative marks which is which, so you can say "la viro posedas la katon" ("the man owns the cat") which is different from "la kato posedas la viron" ("the cat owns the man", even if the cat thinks so). In English the word order determines the Subject-Verb-Object, but in Esperanto you can also say "la viron posedas la kato" (which is similar to "the man is owned by the cat").
To put it simply: you use the nominative case when you do not need the accusative case. Having said that there are already several articles here in Stack Exchange explaining when to use the accusative case and when not. You might want to look at Marcus Cramer's answer to the question When should you use the accusative case?
A more general introduction to the grammatical cases can be found for instance in a Youtube video on the channel Langfocus.
