but why?

It is the informal form of です and there's no right translation for it. ですand だ are phrase "enders" or "finalizers" (in this case I don't know the right term in English) that means 100% of certainty in the sentence information.

i-adjectives have the verb 'be' incorporated in their meaning. That's why these adjectives have a past tense as well as if they were verbs. For instance oishii alone means IS tasty - so adding DA would be repeating IS. Meanwhile desu can be added to be formal.

Seems like Bunpro finally gave up the answer, but there was a fun fact follow up: Caution Unlike な-Adjectives, い-Adjectives must never be followed by だ. This is due to だ only being able to conjugate with nouns (which な-Adjectives originally are). Instead, です is used with い-Adjectives to make them sound more polite. Fun Fact Although using です with い-Adjectives is very common, it is actually incorrect Japanese. As です is simply a polite variation of だ, it follows all of the same conjugation rules as だ. This means that it should only be attached to nouns and な-Adjectives. However, as most Japanese natives do not know this rule, it is widely considered natural to use.