They may not like Phoenix in the summer. They may find that they miss friends and family more than they thought they would. It is hard to start all over building new friendships. Some clients who definitely know they want to retire in another location take the step of buying a house a few years ahead of time and renting it out before they retire. If it is an area where real estate values are rising, this is an excellent way to lock in today's prices and relatively low interest rates.
Some clients really do their research well.
They take vacations in places they think they might want to live, taking time to talk to friends who have moved there. They find out what they like and dislike about their new home. One couple I know did this for several years, checking out Florida, Texas and other southern climates. Longevity is affecting people's attitudes towards retirement. When you consider you may be retired for 30 or 40 years, the whole concept of retirement changes.