i think that running by its very nature is a solitary pursuit. Many, if not most, runners at an event are competing against their own best time, not necessarily racing to place. Unless they are podium material, and at fifth place, that's not podium. So, the emphesis is not about the group at all.
Add to that the fact that runners look forward. After a race, i only really congratulate other runners that were in my field of vision. IF someone was behind me for the whole race, i simply don't know them from Adam.
I've found that you get out of this what you put into it. When i've talked to people at a race, they've talked back. If i didn't take the initiative.... nada.
And running is pretty much a do-it-on-the-fly sport. Not much gear, not much prep. time., not much of anything really. The downside of that is that a lot of runners just want to slap on the running shoes, see how their time stacked up against the personal best, and get home to download their race onto Garmin or stava site or whatever.