“Why are they called ‘apartments’ when they’re all stuck together?” is a joke that has been cited in print since at least 1986 and that has become popular on the internet. The comedian Gallagher has told the joke, although it’s not known if he told it first.
 
Apartments in an apartment building might be “all stuck together,” but each dwelling unit is apart from the other units. Apartments in a large city like New York City might be “all stuck together,” but apartments in a tropical resort area might not be stuck together—both are called “apartments.”
   
“Apartments should be called togetherments” is a related saying.
   
“Why are they called buildings, when they’re already finished? Shouldn’t they be called builts?” is a similar jocular question.
     
   
Merriam-Webster Dictionary
apart·ment noun \ə-ˈpärt-mənt\
Definition of APARTMENT
1: a room or set of rooms fitted especially with housekeeping facilities and usually leased as a dwelling
2: a building containing several individual apartments
Origin of APARTMENT
French appartement, from Italian appartamento
First Known Use: 1641
 
10 September 1986, Sun Sentinel (Fort Lauderdale, FL), “A few questions that may not be answered” by Ray Recchi, Lifestyle, pg. 1E:
Why do we call them apartments when they’re all stuck together?
     
4 July 1993, Dallas (TX) Morning News, “For rodeo relief, try a Gizmo: Unusual clown star punches lines along with bulls” by Dennis Baggett:
“How come they call them apartments when they’re all stuck together? They ought to be togetherments.”
 
Google Books
Cyber Jokes:
The funniest stuff on the Internet
By Doug Mayer
Kansas City, MO: Andrews and McMeel
1996
Pg. ?:
Why do they call them “apartments” when they are all stuck together?
   
Google Books
No. 111 2.7.93-10.20.96
By Kenneth Goldsmith
Great Barrington, MA: The Figures
1997
Pg. 217:
... Why are they called apartments when they are all stuck together?, ...
 
14 February 1997, Charlotte (NC) Observer, Liiving, pg. 11E:
Why are they called “apartments” when they’re all stuck together?
 
30 June 1997, The Interlake Spectator (Stonewall, Manitoba), “Merv’s View: Think about these” by Merv Farmer, pg. 7, col. 5:
Why are they called apartments when they’re all stuck together?
 
Google Books
In Mixed Company:
Communicating in small groups (8th edition)
By J. Dan Rothwell
Boston, MA: Wadsworth, Cengage Learning
2013
Pg. 12:
Comedian Steven Wright asks, “Why is it that when you transport something by car it’s called a shipment but when you transport something by ship it’s called cargo?” and “Why are they called apartments when they are all stuck together?”