maybe because "the gods help those who help themselves" and since karma (the way i understand it) is a balancing of energy it won't act at all unless you acted first ... since its a cosmic thing you can't expect the universe to react in norms and standards of human lifetimes ofcourse.
I wouldnt rely on karma too much to get even, its very true the fysical nature of reality tends towards balance
equilibrium, there's a few other nifty words one could use on the subject, "all things go to the one" (as all things come from the one) its like a mystical explanation of the big bang, the expanding universe and who knows (when it inhales again maybe the contracting universe)
For a moment consider the nature of "divinity" or divine entities, what is "a god" ?
A god would be creature of divine magnitude, something SO huge it would need the equivalent of an electron microscope to even realize we exist (us microbes) , just like you would need a microscope to find out the desk and keyboard you're sitting at is full of life ... its always been there, but you never noticed, you KNOW it since someone once used that microscope and saw it, and you believe it because they are reliable sources of information but if they didnt, you would never know about the microcosmos at your fingertips until you saw for yourself.
Project that upwards to divine magnitude, it would take quite some effort for a microbe to be noticed.
So maybe for karma to kick in you need to make some serious waves ... reality has wavenature after all, up and down across the sinoid, if it didnt do that everything would be dead, still and stale ...
a magical philosophical answer to a mystical question since nobody will know for sure
perception is reality , if YOU believe in karma then its real for you since it is part of existence, that doesnt mean somebody else will, and there's no way to prove or disprove wether it exists or not
the only thing one can know by observation is the universes drive for equilibrium ... something that (metafysisc lol) might actually make this existence the only perpetuum mobile (on a cosmic scale) as entropy drives things outward , and equilibrium pushes back to balance, neutrality and tries to stand still, for that moment of peace but once it gets there, all directions shift and it gets pushed outward again
there i go again, i was just gonna say karma doesnt act , it re-acts :p
newton in the metaverse, at your service :p