Given that you can have anything you want generated in a holodeck, why would anyone ever leave? That is, apart from the need to eat, drink and use the little Starfleet room, you wouldn't have any need or desire to leave.
Aren't holodecks completely addictive?
Yes, Star Trek dealt with that in two separate episodes, ”Hollow Pursuits” (a pun on ”holo”).and the excellent DS9 episode ”It's Only a Paper Moon.” Both of these episodes imply in the title alone that spending time in the holodeck is fun, but ultimately ”holo”.
We have people in our time who are addicted to World of Warcraft, the Internet,.and other forms of entertainment. The only thing that ultimately compels us to step away is the urge to search for a more meaningful existence that is written into our souls.
Are you asking about holodecks on StarFleet warships?
Regulations. You only have X amount of leisure time. Plus, most likely, holodeck use is limited as well.
As far as "why would a civilian with access to his own personal holodeck ever leave", that's not really a SFF question. It's the same as any other addiction.
Some people wouldn't leave - there are people who starve their children playing World of Warcraft, after all. There are alcoholics and druggies. There are people spending 12 hours/day in front of TV. There are people asking 10 questions a day on Stack Exchange :)))
And then there are people who don't.
There are a number of reasons to leave holodecks:
To meet real people, people with different personalities, that aren't predictable because they're created from a programming template. Relationships (all relationships, not just romantic ones) would get boring after time with programmed personalities.
We don't know the limits of holodeck technology, but in the first season of Star Trek: The Next Generation we find there are limits. In 11001001 Riker and Picard are both impressed with Minuet because of her abilities as a character.
There are limited time allowances, at least for Starfleet personel, for holodecks and other people will want to use them, which means limited availability.
Even though, in the 24th century within the Federation, there is no longer any unit of exchange like money, it's still necessary to keep society working, which means real work. For example, Benjamin Sisco's Father is a chef who runs a restaurant. There would be no food for him to prepare if it weren't caught or raised or grown as crops. (And somebody has to fix and run any machines that help with these functions!)
Exploration. This is the very purpose of the Enterprise (supposedly -- when they're not shuttling delegates back and forth or worried about if they should cross the Neutral Zone). Without leaving the holodeck, they couldn't make contact with new races and new people.
Holodecks can't maintain a program for but so long. In Homeward, when Worf's brother has a tribe beamed into a holodeck to save them when their planet will be destroyed, they find that after a while, the holodeck has trouble maintaining the hologram continuously.
Most people want more out of life. You could have as many holowhores (or holostuds) as you want, but if you know that they're programmed to easily tumble in your bed, it's not going to be satisfying after a while (even if the sexual experience can be well simulated holographically) because there will always be the knowledge that you haven't worked at developing a real relationship with a real person.
As good as a holodeck can be, there are infinities of possibilities they can't replicate completely or perfectly from real life.
I think it's because as much as Gene's vision was for there to be no "money" in the Federation, there is still a cost to anything, especially on a deep-space post or a starship.
I think it is more a matter of 'humanity has grown up a bit' issue. Consider now: We are stuck on a single, overpopulated planet, with too many people stuck in limited lives, boring jobs, etc. Hence why so many people today consume so much time in booze, drugs, video games, Internet activity, cosplay, etc. - reality just doesn't satisfy. In the Star Trek universe, interstellar travel has opened up great vistas for people to have meaningful lives exploring the universe, building new societies on other worlds, expanding the frontiers of knowledge, and so forth. Hence the urge to retreat from reality has largely evaporated. [not to mention that a lot of the 'dead weight' got wiped out in World War III, with the survivors needing to focus on survival and rebuilding, not having time for this nonsense...]
I am sure that there are plenty of people in the Star Trek universe that do use holosimulations almost exclusively in their lives. It is often described as a simulated laboratory. Remember though with modern ships and Star Trek ships there is a limited amount of resources, in particular power. So while running simulations for fun and training are advantageous, particularly when spending years in space. It would not at all be feasible to run the entire ship from a holodeck even from a purely resource oriented aspect.
I have vague recollections of this throughout the series, but remember one particular TNG episode where the Enterprise is being drained of power and LaForge has to get Picard's approval to continue a simulation on a holodeck to figure out the solution. This would imply a significant resource drain to maintain and simulation.
