“That’s show business” (also “That’s show biz” or “That’s showbiz”) means something similar to “That’s life!”—there’s bad along with the good, and nothing is easy. “Oh, that’s show business” was cited in print in 1922 and the saying has been popular since at least the 1920s, if not earlier.
 
According to a 1957 account, songwriter Irving Berlin (1888-1989) told the young singer Sophie Tucker (1886-1966): “Kid, that’s show business.” “And that’s show business” was a chapter of Tucker’s autobiography, Some of These Days (1948).
 
The magazine Playgoer Monthly explained in 1949:
 
“For when we say ‘That’s Show Business,’ we mean a man or woman facing up to tragedy and disaster, and still going on with the show. Or an understudy achieving a big success, only to have it hushed up or shrugged away, to satisfy a jealous ‘star’ or a disinterested management. We mean the hundred-and-one blows and heartbreaks that we in this mad profession have to face and overcome.”
   
   
The Free Dictionary
that’s show business (for you)
That is the way that life really is. (Also with biz and show biz.)
 
Google Books
December 1922, Illustrated World, pg. 634, col. 1:
“Oh, that’s show business,” replied Terrill and Warner to New York theatrical, men who had journeyed out to see how the big top lads would handle an extraordinary and unprecedented situation, and who were moved to express their admiration unreservedly.
 
15 January 1928, Sunday World-Herald (Omaha, NE), Entertainment Section, pg. 8, col. 1:
A little discouraging to those of us who have admired him for what he has done. But that’s show business.
 
7 August 1929, The Daily News (Huntingdon, PA), “On with the Show” by Arline De Haas, pg. 5, col. 1:
“So go on, and give them everything you’ve got. That’s show business, Kitty. Do it for both of us.”
 
Google Books
Two Plays:
Not for children and Between two worlds.
By Elmer Rice
New York, NY: Coward-McCann, Inc.
1935
Pg. 5:
You see, the last couple years, I don’t seem to be getting the breaks. Well, of course, that’s show business.
 
Google News Archive
9 February 1937, Berkeley (CA) Daily Press, “Elaine (Barrie Barrymore—ed.) Returns As Play Closes” by United Press, pg. 11, col. 3:
“But that’s show business, good with the bad,” she said.
 
Google News Archive
28 April 1942, Middlesboro (KY) Daily News,  “Frantic Weekend” by Edmund Fancott, pg. 2, col. 4:
Baldy gave a grunt of disgust. “That’s show business.”
     
OCLC WorldCat record
Some of these days.
Author: Sophie Tucker
Publisher: London, Hammond, [1948]
Edition/Format:   Book : English
Contents:
And that’s show business.
 
Google Books
Playgoer Monthly
Issues 479-499
1949
Pg. 212:
For when we say “That’s Show Business,” we mean a man or woman facing up to tragedy and disaster, and still going on with the show. Or an understudy achieving a big success, only to have it hushed up or shrugged away, to satisfy a jealous “star” or a disinterested management. We mean the hundred-and-one blows and heartbreaks that we in this mad profession have to face and overcome.
 
Google News Archive
7 March 1957, The Ledger (Lakeland, FL), “Dorothy Kilgallen Reports On Broadway,” pg. 4, col. 7:
TO ONE FAMILIAR of the weird backstage machinations of a Broadway musical, it’s almost as if someone had put the whammy on her because—somewhere, out of town—she got too much applause to suit another member of the cast with bigger billing and more power. It happened to Sophie Tucker when she was a youngster, and when she cried to Irving Berlin he told her, “Kid, that’s show business.”
 
OCLC WorldCat record
That’s show biz!
Author: Ray Anthony; Diane Hall; Annita Ray; LeRoy Anthony
Publisher: [Scranton, Pa.] : Capitol, [1961]
Edition/Format:   Music LP : Jazz : English
 
OCLC WorldCat record
That’s showbiz
Author: Tom Cowan; Phil Noyce; David Huggett; Don Connolly; Chris Fillam; All authors
Publisher: Sydney : Australian Film and Television School [production company], 1973.
Edition/Format:   Film : Film Visual material : English
Summary:
Summary:A comedy which details the rise and fall of a failing outback vaudeville troup who import an infamous city strip-tease artist to enliven their act.
 
OCLC WorldCat record
That’s show business : the rules of exhibiting
Author: John Hemingway; Video Arts Incorporated.
Publisher: London : Video Arts, 1990.
Series: Briefcase booklet.
Edition/Format:   Book : English