I find a lot of people, even those who don't tend to have other religious/spiritual beliefs tend to hold on to karma (if not necessarily in the Hindu/Buddhist context of an afterlife, then still in some metaphysical form). I think that maybe it makes people feel good to believe that justice eventually comes for everyone, good, bad, or otherwise.
Me personally, I don't believe in karma in any metaphysical sense of the word. I do contest that people tend to gravitate toward those of a similar nature to themselves (birds of a feather...). Thieves with thieves, bullies with bullies, empathic with the empathetic, etc... In this way, people tend to get what they give. But sometimes, maybe even oftentimes, good things come to bad people with no equitability, and bad things happen to good people with no recompense. Fair? No, but no one promised that.
Sort of. It all comes back to the 'do unto others' philosophy.
We are a social species and so tend to gravitate to people who have similar values to ourselves.
Sometimes shit happens to good people and sometimes good happens to shit people. But generally we make our own luck and for the most part, if you are good to others, it gets reciprocated.