There can be many reasons. Natural inefficiencies like people falling asleep, getting tired or bored, calc mistakes and corrections, holidays and etc.
Then we have the wait period between two teams getting ready to review and evaluate. And so on and so forth.
The way I look at it is, we are only investing our money. But they are investing their money (though significant chunk belongs to investors), time, emotions and careers. So, they are sure delay is not their best choice, rather it can be an unavoidable or sometimes a necessary evil