Its likely that Viruses like COVID-19 have come and gone over time, and been completely unnoticed.

The difference being now that we have much older and elderly populations, often with chronic health conditions and deficiencies, kept alive 'artificially' by modern healthcare.

For example, approximately 50% of COVID deaths are to people in nursing homes. Without this population, the virus would be put down as regular influenza/pneumonia, albeit an unusually virulent strain.