It is hypothesized that there is a rough upper limit to the number of people our brains can naturally conceive of:

https://en.wikipedia.org/wiki/Dunbar%27s_number

Of course, with effort, we can be mindful that in fact we are surrounded by millions of strangers, but it's not an instinctual emotional reaction. When in environment like sitting at home staring at a screen, it's easy to forget to do that cognitive work.