They may like the convenience of the location, the qualities of the neighborhood, their actual neighbors, the physical layout of their home, or other factors that make continuing to live where they are highly desirable. Conversely, their present home may not be their idea of the perfect home but it continues to serve their needs. Remaining in it is easier and less work than trying to find something else and then moving into it.
Even when their current home does not address all of their needs, it still is their home. They know what they have, and they don't know what they will get in another home or if they will be any happier living in it. From a practical standpoint, it often makes more sense just to remain where they are.
There comes a time in life when moving is not fun. While people have moved out of their parents' home, out of their college apartment or dormitory, out of their first apartment, and other places along the way, they've discovered that the days leading up to a move, the move itself, and the days immediately following a move can be quite stressful and generally disruptive to their lives.
At younger ages, moving can be fun and is generally anticipated with some degree of excitement and adventure - moving for adventure, to explore a new city, for an employment opportunity, for larger space, for a more prestigious address, or to gain a new start.
Later in life, however, moving isn't as much fun anymore. Some people would argue that moving is never fun even though it can be exciting or necessary.
Moving means that someone's present home - unless it's a rental - has to be readied to be shown to potential buyers, advertised for sale, and then sold. For several weeks, if not months, prior to the actual sale there is a getting ready period where that home is no longer something they enjoy returning to every night. They must paint, fix, repair, and clean it to get your home ready for potential buyers to view it in its best light. Actually, this home begins to feel as if they are just a guest because essentially that is what they are once the decision has been made to sell it.
Moving means putting all of your belongings - and memories - into boxes, crates, or cartons and preparing it to be shipped to the new address. This is true whether someone is doing the move themselves or having it done by professional movers. Also, things will get broken and some items will get lost. The best way to avoid this is to refrain from moving.
In addition to the expense of moving and the general anxiety and disruption of preparing to move and then actually doing it, people can age in place in their current home and keep it as it is or put some of that money they were going to spend anyway into making their home safer and more comfortable for them to continue living in over time.