Very few of the failed teams did their own research. Most didn’t do any research. Instead, they made up the personas out of whole cloth and what they imagined their users to be like.
Assumption! What you would do or what you think a group of people would do based on your sociologic experience is a quick, easy and cheap way of doing things - but will prove a problem in the long run. Those shortcuts might have worked doing homework in high school, but it will never apply to product design and UX research as it is simply not the real world... even if your assumptions are close!