Cities aren't "tribes," they're far too large. In fact, they're tribe destroyers. From an evolutionary perspective, interaction with other people was meaningful: either you were interacting with a tribesman (basically a member of extended family) or with an outsider. In neither case was "indifference" a desirable response (outsiders evoke competitive fear or interest). In a city you interact with so many different people on a daily basis that almost all human interactions are, by necessity, marked by indifference.
We're not designed to simply ignore people. However, more and more that's what we are required to do.
I carefully considered your position and partially agree with some aspects of it.
If you are correct, that would imply that over a very long term, the total level of "drama" would be decreasing over time in the average life as city size increases etc. I disagree. Also a decline in actual drama below some instinctual drama-thermostat in the brain would result in increasing levels of interest over the centuries in artificial drama (plays, gossip, TV, social media, maybe religion). I'm not seeing that either. I would theorize that changing technological tools for artificial drama have changed the method of stimulation but not the quantity, although I don't have enough data to know if I'm right or wrong. Its rare in the social sciences to have the chance for a hard numbers science experiment like this, which makes this topic interesting. In theory, for example, over the past generation, interest in traditional cultural plays in China should have exploded upwards thru several orders of magnitude. On the other side, there has been explosive growth (perhaps a fad?) in social media.
I would also make a very small correction to your last line:
"However, more and more that's what we are required to do."
should have two words added:
"However, more and more that's what we are required to appear to do."
For example, I would theorize based on this that the total amount of "checking out the ladies" is constant among males of a similar hormonal level across all cultures, what varies is the resulting behavior after paying attention (some places catcalling is the law of the land, some places no one whistles, ditto staring, and many other reactions, it only varies in male reaction, not in the quantity of male attention)
I would agree that the prevailing behavior in the west is to pretend to ignore, while paying very close attention. Don't make eye contact with the pan handler but keep an eye on him in the unlikely event that he pulls a knife. She wore that so you'd check her out, duh, but not to make you outright stare, duh. Pretending to ignore is somewhat different than actually ignoring.
We're not designed to simply ignore people. However, more and more that's what we are required to do.