Why is "Uso Da!" used in Higurashi? I haven't heard  &quot;Uso Da!&quot; used to mean &quot;Liar!&quot; in any other anime from what I can remember, and when I type the phrase into google, it seems to exclusively be a Higurashi thing. Why is this?  If it were just a dramatic word that's not really used in real conversation, which is common for media, I'd expect to hear it more elsewhere in other anime too. I'd love to hear exactly why uso da seems exclusive to Higurashi, thank you ^^
だ is really just there for emphasis, and would likely be there for emphasis in any other media or show.
From the article:

The most basic functions of だ are to mark nouns and な-adjectives as positive (true), and present tense (relevant to now or the future). だ is not used with word types that have their own way of telling us their tense or whether or not they’re true, such as verbs and い-adjectives.

So if you heard someone say　&quot;嘘{うそ}だ！&quot;, phonetically it's no different than if someone said, &quot;You liar!&quot;
As with natural speech, there is always nuance in how people speak certain phrases.  This example is no different, and the real answer as to why this form is used over others is a bit beyond my completely non-native understanding to assist you with.
I do hope that this can give you some enlightenment as to why it is the way it is, though.