Frankie Muniz from Malcom in the Middle retired from acting in 2006.
He, like some others mentioned above, was a child actor, and I think quite a few retire on reaching adulthood. I think that transition is difficult; some kids get roles because they look cute as children but as adults, they lose the cuteness factor. And also, it’s a really brutal competitive business.
There was a documentary called Showbiz Kids on HBO last year (directed by Alex Winter of the Bill & Ted movies) and it featured quite a few former child stars, some of whom are still acting as adults and some who got out of the business. It went into some of what the kids experienced (including sexual predators in the business).
We should mention all the stars who quit their careers to fight in WWII. Not just making war films like John Wayne, but actually fighting and not acting.
David Niven was working in Hollywood when the war broke put. He re-enlisted, and took part in the invasion of Normandy among other dangerous assignments.
Jimmy Stewart was already a big star when he joined the Army Air Corps in 1941. He flew 20 combat missions in B-24s over Germany, becoming a squadron commander. He continued flying in the reserves after the war, and eventually became a brigadier general.
Clark Gable was a huge star when the war broke out. His wife, Carole Lombard, was also a star and was killed in a crash returning from a war bonds drive. He quit acting and joined the Air Corps at age 43, eventually flying five combat missions.
Alec Guiness was already an actor when the war broke out, and he enlisted in 1939 and commanded a landing craft during the invasion of Italy.
There were other stars, like Audrey Hepburn and Josephine Baker who were involved in the war as part of the resistance or in spying, but they maintained their entertainment careers for cover or to raise funds while doing it.
I have the utmost respect for those people. They could easily have sat out the war or spent the war raising bonds or making propaganda films and such, but they not only volunteered, but they did so with no special dispensation for their fame and fortune, and some wound up in some of the most dangerous jobs of the war. Bombing Germany in a B-24 was extremely high risk, for example. No soft details for this bunch.
Adam Sandler is absolutely capable of delivering seriously good performances - he nailed it in Uncut Gems, The Meyerowitz Stories, Reign Over Me, Funny People - his comedic roles aimed at younger audiences may indeed be annoying, but he is not the one-note actor you might think him to be.
Zeppo Marx quit show biz to become an agent.
If by agent you mean a theatrical or movie agent, that’s not exactly quitting show business, is it?
He stepped away from acting. He also worked in real estate.
Although his acting career is now considered a bit poor, at the time (the later) President Ronald Reagan was considered a ‘star’ worthy of top billing.
TCMF-2L
I think there’s a few categories here:
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Child actor. Got to adult. Stopped. Kind of works for teen actors, some of those stopped in early 20s.
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Female star, off to have children, not interested in coming back
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Retiring actor, got old, quit before they got bored and took it easy.
Perhaps bereavement too, taking care of the family now the partner has gone might feature too.
Yes, and all those aren’t really remarkable.
- Pushing-middle-age actor: Wasn’t really that good an actor and ceased getting good offers, so decided to live on their savings for the moment and go into another business if it was necessary to make some money.
- Child actor: Decided to get things done like college, marriage, having children, etc., so may decide to go back to acting later.
- Middle-aged actor: Discovered that directing or writing movies is more interesting than acting in them, so took those kind of jobs with only occasional acting in addition.
- Middle-aged actress: Was very popular until she reached a certain age (usually somewhere around 40), at which point she stopped getting good offers for movie or TV roles, so she started to do more stage work, or just retired entirely.
- Actors that dropped out due to ongoing issues of whatever nature.
Tippy Walker apparently was a bit fragile and consequently often came across as rather “difficult to work with.” Her teenage relationship with middle-aged married director George Hill didn’t seem to have done her any favors either. Her retirement seems more a matter of just no longer getting serious offers.
Her teen co-star Merrie Spaeth from The World of Henry Orient did a Peter Ostrum and just walked away from the industry, eventually becoming a well-known Republican consultant.
ETA: Oops, didn’t see this was an older thread when it was linked elsewhere. Oh, well - just a 1 month bump 
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Jan Smithers (Bailey Quarters) worked fairly steadily until she married James Brolin in 1986, then left the business entirely.
There must have been a fair few whom the movies stepped away from, as it were, especially when sound came in. Clara Bow, for one. For others, it’s hard to know which had the greater loss of interest: Pola Negri?
I can remember the interest when Veronica Lake was found to be waitressing in a diner.
Okay, I watched Uncut Gems, and I’ll admit Sandler has some chops.
That pair of directors (the Safdie bros) got a great performance out of another actor I was meh about: Robert Pattinson (in Good Time).
IIRC, he became a born-again Christian and that was when he made that announcement.
James Spader was a very popular, rising star in the 80s and 90s, and he scaled back his career to focus on raising his children. IIRC, he did a lot of stage work, and locally (to his area) shot movie and TV roles, and then made a comeback a few years ago with a TV series, again IIRC on Showtime.
TYVM. I obviously didn’t check IMDB; I just knew he dialed down his career for a while and then made a comeback.