Funnily enough, according to some South Korean news, the Korean government deliberately postponed having a final written deal, or at least that's what some news articles claim, because it was such a horrible deal for Korea.

I think there's an implicit understanding that having a written deal would be worse for Korea because the deal will bind Korea and restrict its options, while Trump is a crazy guy and can and will do anything he wants, whether he signed something or not. I.e., any written deal with Trump has negative worth.

Such is the state of global trade negotiation these days.