Realising the problems of advanced planning means you need to spend more time, not less, on working out what you are trying to do.
Then why does every modern startup do agile development instead of spending more time on planning?
Well, probably mostly because it's trendy. But as for why people who choose to do agile development for sensible reasons do so, I suspect it's because doing planning and data collection in such a way that they inform one another has better results than planning in the absence of data or data collection in the absence of a plan.
Why do you ask?
Another month, another rationality quotes thread. The rules are: