Why do i have to wait to exchange for rewards in Hilichurls' machine workshop?

Because there is a limited amount, and it almost always taken by bots within seconds. I won’t bother with this if I were you.

All rewards are limited. Don't worry, you don't need to wait, all rewards will be hoarded by bots in less than a second after refresh.

It is a great way to get mora and XP books

I wouldn’t waste your time with trying to get those rewards. Bots will have them gone within seconds of the timer finishing.

Wish they would remove the limit on these rewards... Keep the personal limits per item, but remove the global limit so everyone can get something.