Before a certain time of day in most cities (varying a bit on location, even within a given metro), it is quiet; that quiet time is not meant to be disturbed. Unless you are actively working, then there is no reason to talk. Simple nods of acknowledgement will do in most cases, but interactions should be kept simple as possible. Other morning dwellers typically are there by a choice driven by a desire for peace, so let them have it. Similarly, whistling, playing music, or otherwise generating unnecessary noise is frowned upon. This rule does not apply to birds, who, in an ideal morning, supply the only soundtrack.
Even the act of acknowledging others is questionable. Should one make eye contact with a passer-by? Generally, it is OK to pretend they are not there, and this will be reciprocated. The morning is used to pretend the world is a bit more pleasant than it is, why ruin that? In cities and times where zero people are normally found even during the heat of the day, a "Hello" might even be ok, if only to show that you are not a killer making a move.
These rules apply to those enjoying the morning, but not everybody awake during the absurd hours is enjoying the morning. Some people are working, and the rules do not apply. Some people are on their way to work, and the rules might apply, especially if that is by choice to leave early. Some are actually experiencing the late night, where one can say "rules, schmules" and carry on with being a reveler. Knowing that you are truly breaching the hazy line that forms between night and day adds to the mystique of the morning.
Most who wake up early enough know these rules and abide by them. A secret society of those who embrace the time before the first dog walkers start the day forms where ignoring others is encouraged. Corona has created a pool of newcomers, some of whom seem to need to be informed.,