Because that is the definition of R0. Once people are aware and change their behavior R changes as well and is not readily translatable to a different setting.
From the article:
Dr Feigl-Ding explains that R0 is the “R reproductive number at time 0 before countermeasures”.
He points out that this is not the R(effective) at current time under mitigation measures such as distancing and testing, tracing and quarantine, which are expected to slash chains of transmission.