Why? I mean, think about it. Donner was an extremely intelligent, accomplished director. He made multiple classic movies and hits. And he clearly used an entirely different artistic style for the Smallville scenes that was grounded in the filmmaking aesthetics of the 1950s for the 18-year-old Clark scenes, and of the 1940s for the Baby Clark scenes. And again, this was the late '70s -- it's not like they didn't know that someone turning 30 in 1978 was a child in the 1950s. Reeve was right there to tell them (in fact he didn't turn 30 until 1982). And the math lines up with Superman's line about Krypton exploding in 1948 if we take that to refer to when both his ship and the light from the explosion arrived on Earth.
ETA: Also, bear in mind that nothing about the writing of the Smallville scenes requires it to be the 1950s. The things that mark it as the 1950s are all matters that would be under the director's control -- the costumes, the cars used, the hairstyles, the music played on the car radio. It's the aesthetic choices of the director, not the writing, that evokes the 1950s, nothing in the actual script. End edit.
This wasn't just a careless line. This was deliberate anachronism to create a particular artistic effect.