If you can get a bunch of people into a room and plan out what everyone on a software team is doing for the next three months, there's a pretty clear cap on how creative or adaptable the work can be. What is everyone doing for the rest of the quarter? "Executing"? Just filling in the "obvious" implementation details to program exactly what they're told?
Getting a clearer picture of the next 3 months should be a piece of cake. As a bonus once it’s planned you should get to focus without a sudden shift in priorities.
(Note: quickly, not suddenly. Bad performance in market validation should never come as a complete surprise. It always slowly dawns on you over the course of days/weeks.)
Another reason for changing priorities quickly is that the specification drawn by the product team happens to be technically very complex because it interacts in unforeseen ways with other functionality, and this is discovered early in implementation.
Basically, in product development, you're constantly running into things that either provide less value than expected, or cost more than expected. That's when you should do something else instead.
Once you have a great idea, yes, you should sink all of your effort and focus into that to get it to market quickly. But the idea is that if you start cautiously and change your mind early and often, you can actually implement the winners with blazing speed later without looking back.
Which is most orgs I've worked with.