“Plays aren’t written, they’re rewritten” is an old theatre adage meaning that many changes are usually made during the rehearsal process, before the play officially opens. Dion Boucicault (1820-1890), a popular playwright and actor, has been credited with the saying since at least the 1900s, but it’s not known when Boucicault said it.
 
“Musicals aren’t written, they’re rewritten” is a popular extension of the adage. Other types of writing have been used in the saying (“Novels aren’t written,” “Songs aren’t written,” etc.).
 
   
Wikipedia: Dion Boucicault     
Dionysius Lardner Boursiquot (26 December c. 1820 – 18 September 1890), commonly known as Dion Boucicault, was an Irish actor and playwright famed for his melodramas. By the later part of the 19th century, Boucicault had become known on both sides of the Atlantic as one of the most successful actor-playwright-managers then in the English-speaking theatre. The New York Times heralded him in his obituary as “the most conspicuous English dramatist of the 19th century.”
   
17 January 1909, Springfield (MA) Republican, “Plays Really Rewritten,” pg. 21, col. 3:
“Plays are not written, they are rewritten,” once remarked Dion Boucicault.
 
Google Books
The Footlights, Fore and Aft
By Channing Pollock
Boston, MA: R.G. Badger
1911
Pg. 237:
Many plays—even most plays—are substantially altered at rehearsal. Dion Boucicault, the great Irish dramatist, said: “Plays aren’t written; they are rewritten.”
 
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The Career of Dion Boucicault
By Townsend Walsh
New York, NY: Dunlap Society
1915
Pg. 103:
All of which goes to prove the truth of one of Boucicault’s pet aphorisms: “Plays are not written; they are rewritten.”
 
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The Technique of Play Writing
By Charlton Andrews
Springfield, MA: Home correspondence school
1915
Pg. 223:
(Digest of Dramatic Rules—ed.)
28. “Plays aren’t written; they’re rewritten.” “It’s a wise author that knows his own play on its first night.”
 
5 March 1916, New York (NY) Times, “Dramatizing a novel is a chastening experience” by Marian de Forest, pg. X8:
... Dion Boucicault (was it Dion Boucicault?) knew what he was talking about when he said: “Plays aren’t written; they are rewritten.”
   
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October 1917, The Green Book Magazine, “Trying It on the Dog” by Channing Pollock, pg. 580:
You may believe it or not, but in the three weeks we were on the road, he never once informed me that ‘Plays aren’t written: they are rewritten!’”
 
The origin of this axiom is shrouded in mystery. Popular opinion credits it to Augustin Daly, or to Dion Boucicault, but whoever invented the phrase, it has become the theatrical equivalent o “Virtue is its own reward” and “You can’t make a silk purse out of a sow’s ear.”
   
Google News Archive
5 September 1937, Pittsburgh (PA) Press, “Slave to Art” by Kaspar Monahan, society section, pg. 6, col. 8:
Plays aren’t written —they’re re-written.
   
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The World in His Heart
By Josephine Daskam Bacon
New York, NY: D. Appleton-Century Co.
1941
Pg. 60:
They wrote a play, and a fellow that did bits in the group-theatre where Manuel had a pull, was delighted with it and backed it — verbally — for Broadway. Writing it, rewriting it (plays aren’t written; they’re rewritten) casting and recasting it, waiting in managers’ offices about it, telegraphing and lunching about it, occupied a full year ...
 
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Not as a Crocodile
By Arthur Marx
New York, NY: Harper
1958
Pg. 207:
Of course, their play was a little rough for Broadway in its present form, but, as I explained to Steve and Andy later, when we were tucking them into their beds, “Plays aren’t written, boys —they’re rewritten.”
   
25 October 1967, Oregonian (Portland, OR), “TV Writers ‘Re-Dream’ Old Dramas” by Lawrence Laurent, sec. 2, pg. 7, col. 2:
A short satirical item in the current issue of Teleivsion magazine bears the headline, “Great TV shows Aren’t Written: They’re Rewritten.”
 
This is in contrast with a remark by the late columnist George Dixon when he was introduced to a “television writer.” George’s comment was a question: “You mean that someone writes that stuff?”
 
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Fun While It Lasted
By Barnaby Conrad
New York, NY: Random House
1969
Pg. 275:
Remember, books aren’t written, they’re rewritten.
 
14 August 1989, Philadelphia (PA) Inquirer, “(Not) the End: London’s stage people don’t hesitate to rewrite plays,” pg. D8:
“It finishes on a high,” said Black, who went on to quote his American colleague, composer-lyricist Stephen Sondheim: “Stephen says, ‘Musicals aren’t written, they’re rewritten,’ and this is very much the case.”