Just like SARS-CoV-2 wasn't a thing a year ago, but crossed over to humans late 2019, the 1918 Spanish flu virus did indeed only start infecting humans in 1918, independently of other viruses. 1918 just happens to be the year it (most likely) crossed over from an animal to a human.
Assuming generously that you are right about this, why (for example) are viruses crossing over in the other direction - from humans to animals - never considered important?
We might only care about the part of viral evolution and epidemiology that we closely observe or are immediately impacted by, but that does not mean the other aspects of evolution are independent.
Just like SARS-CoV-2 wasn't a thing a year ago, but crossed over to humans late 2019, the 1918 Spanish flu virus did indeed only start infecting humans in 1918, independently of other viruses. 1918 just happens to be the year it (most likely) crossed over from an animal to a human.