Earlier quoted context omitted.
The Paramount Decrees way back in the 1940s/1950s: that's why Hollywood studios cannot produce movies and own the theaters which exhibit them. It's also similar to the (much more complex) reasons TV Service Providers (DirectTV, Spectrum, XFinity et. al) are separate from TV Networks, and why you don't see Disney trying to buy, say, DirectTV. Of course, streaming upended almost all of that.
You seem to be correct about the studio/theater bit, but Comcast owns both NBC and Xfinity, so clearly that bit of intended separation ain't working.
But then there was a drastic change in approach to anti-trust during the Reagan era:
"Bork argues that the original intent of antitrust laws as well as economic efficiency makes consumer welfare and the protection of competition, rather than competitors, the only goals of antitrust law. Thus, while it was appropriate to prohibit cartels that fix prices and divide markets and mergers that create monopolies, practices that are allegedly exclusionary, such as vertical agreements and price discrimination, did not harm consumers and so should not be prohibited."
"From 1977 to 2007, the Supreme Court of the United States repeatedly adopted views stated in The Antitrust Paradox in such cases as Continental Television, Inc. v. GTE Sylvania, Inc., 433 U.S. 36 (1977), Broadcast Music, Inc. v. CBS, Inc., NCAA v. Board of Regents of the University of Oklahoma, Spectrum Sports, Inc. v. McQuillan, State Oil Co. v. Khan, Verizon v. Trinko, and Leegin Creative Leather Products, Inc. v. PSKS, Inc., legalizing many practices previously prohibited."