how do i trade shiai tokens to people if it is possible? when i go to safe trade there's no option for shiai token, only tc and other items.

not possible

You can't trade them with others. All you can do with these is exchange them for items in the torishop.

the question is why cant we trade shiai tokens?

I suspect it could be possible to farm the tokens through the 7th day login reward feature. While yes, it'd be a hassle to pull off, I think it's for the better. You can still buy items for the tokens and then transfer those items to whoever you desire.