Because we didn't do our manned program in a reasonable progression, else we'd have a mass-driver on the moon that could ship raw materials back to earth orbit for TONS cheaper than any earth-to-orbit price.
To be fair, detailed research concerning the use of lunar resources to fabricate space colonies, powersats and other infrastructure (e.g., employing "achromatic trajectories" to accurately guide mass-driven chunks of regolith into catchers stationed at L2) only began in earnest during the early 1970's with Gerard K. O'Neill and the L5 Society.
Assembly in orbit, and in large part fabrication in orbit, is probably inevitable, and not just because every science book for the last 50 years has stated it as a matter of fact. You don't build interplanetary spacecraft in a gravity well, when they don't have to perform there.
Gene Roddenberry need not have bothered with reading astronautical engineering texts to apply the concept to
TOS, considering that Disney's
Man and the Moon and Pavel Klushantsev's
Doroga K Zvezdam ("
Road to the Stars") - both produced in the late 1950s - featured space station components being launched into orbit by cargo rockets. Even if GR somehow managed to miss seeing both films on TV, Robert Justman may have mentioned the idea during a production meeting considering he was an assistant director on
MatM.
TGT