Why can't だ be used after an I-adjective?

Question

I have been constantly told that だ can not be placed after an I-adjective. Why is that?

Answer 1

The い ending that is already attached to the い adjective means "is". The だ is a slang version of "is" so attaching it to an い adjective would be "is is". The な adjectives need the だ because they don't have the pre-attached "is".

Answer 2

Short answer is that I-adjectives already have a sense of "to be" already built in.

In a more complex explanation, NA-adjectives evolved from NARI and TARI adjectives in Classical Japanese. NARI and TARI were copulas, the predecessors to the modern DA/DESU. Since NARI and TARI became DA/DESU, that was preserved.

I-Adjectives came from KI and SHIKI adjectives. As with their modern counterparts, these adjectives also have a sense of "to be" embedded within them.