If no one wants to complete those tasks in real life, or there are conditions that change the nature of those tasks in real life, your user study results will not reveal those things
I think this highlights the most prominent weakness of these user studies. Despite that weakness, it seems like usability tests can be very valuable and worthwhile when designing an interface. It seems like all of the design methods we learn about are strong in one way and weak in another, so it's really starting to click for me why we should do so many types of tests and use different methods.