The art in poetry is poetry, which includes all forms of it, so the poet isn't limited to any specific form, and many did write in different forms. Similarly unclear what was arbitrary about oil paints, what was a similarly colorful alternative without such limits?
>The art in poetry is poetry, which includes all forms of it
Only in abstract - before you get to do it. When you do start to write a specific poem this doesn't hold anymore, and a big part of the art is fitting the form you chose.
> >The art in poetry is poetry, which includes all forms of it
Imho the "art in poetry" begins when I had that dinner with that fascinating woman, and I had the "porch test" and in that test it was raining, and I came up with a haiku. But not (only) on the haiku itself.
I mean poetry is an arrangement of symbols, generally symbols that are related in their representation: assonance, dissonance, rhyme, meter, stress, meaning…
The poet is limited to symbols. And every poet comes up against these limitations.
But the symbols aren't an arbitrary limitation - for example, using non-language symbols would mean that he will simply not be understood, so the understanding is drive by the need to communicate