I think (the collective) we don't see as much genuine 'standing up for' (vs. the 'for status/reward' versions described) is... well, it's depressing, but true:
...people don't necessarily know or recognize it if they aren't in the group being targeted.
There have certainly been times I've been in the group that's being targeted and am oblivious, and I see a fair bit of this go around as well.
It is non-trivial if not actually impossible to stay on top of all of the creative, twisted, fucked up ways people express gross and horrifying bullshit, even among genuine allies -- as in, the people who absolutely would speak up for the right reasons if they recognized what's happening.
This is legitimately frustrating for all parties except the assholes, which is essentially the worst possible outcome for decent human beings and anyone simply trying to go about their life minding their own business unmolested by assholes.
I wish I knew what to suggest here, beyond 'grab someone you know who either is familiar with what gross is happening, or grab someone you know who isn't an asshole and be (frustratingly, I know, believe me, I really do) willing to provide a short-form (one-two sentence) summary of the thing. (ex: 'the word <slur> is a slur used to dehumanize <group>' should be more than enough for a genuine ally without a long-winded dissertation/extensive emotional labor required to clue them in.) It's a shitty solution, but it's the only one I've got. 