Because those diseases have been circulating amongst the human population for thousands of years (or longer), and there is a level of population immunity that k

Because those diseases have been circulating amongst the human population for thousands of years (or longer), and there is a level of population immunity that keeps large outbreaks from occurring. Also, you aren't even aware of most of the viruses you're infected with on a regular basis, because you aren't looking for them, and immunity you've built up over a lifetime reduces the severity of symptoms. For all you know, you've been infected asymptomatically / minimally symptomatically with any number of common respiratory viruses in the past year, and written it off completely. The big difference for SARS-CoV2 was that the entire global population was immunologically naive in 2020 (maybe; there's evidence for some level of immunity due to common-cold coronaviruses). This is rapidly changing.