>>104146913They're a very old company and they've been consolidating assets for a long time. With tv in particular, it wasn't until 1993 that the fin-syn laws ended and it was possible for companies to own both a production studio and a network (hence the rush of "new" networks showing their own-brand in-house shows made by old studios in the 1990s onward).
Back in the 1960s, someone thought it was a good idea to make a Batman show. The rights ended up (as was the style at the time) shared between three companies and I think some individuals as well. Because the contracts were old and nobody had read them in years or had the skill to interpret them, nobody really knew who owned the Batman show, so it couldn't be released on DVD until 2014. But the film and tv rights themselves - the right to make new tv shows and movies - reverted to WB sometime in the 1980s allowing them to make 1989's Batman in-house.
Likewise with Superman. The movie rights were with the Salkinds for Superman, Superboy, Supergirl (and possibly Supermutt, Krypto, that horse), and the Salkinds got two good movies out of them, two middling to bad movies, and sold the Superman rights but clung to Supergirl (for some reason) and Superboy, which they developed into a tv show which was quite popular. The Superman rights went to Cannon, who ended up bankrupt shortly after and then on to WB; the Superboy and Supergirl rights stayed with the Salkinds until 2005. There were protracted legal battles (in which WB was not always in the right or acting in good faith).
But to answer your question: because there were very strong antitrust protections in place for tv broadcast, Reagan killed them, the studios consolidated, the number of studios shrank drastically, the output shrank too. Those protections were originally put in place to end the Studio System (prior to tv becoming mainstream) whereby studios would block-book theaters with their own in-house produced and distributed movies. It was a bad system.
